For generations of boys, sexual abuse was part of the everyday cruelty of boarding school. In this painfully honest report, writer Alex Renton confronts the demons of his past at Ashdown House, where some of Britain's most powerful men were also educated – and reveals the scale of the outrage about to engulf the private education system

If Ashdown House's pretty Georgian facade reminds you of Washington's Capitol and the White House that's because the architect, Benjamin Latrobe, had a hand in those, too. It is an excellent look for the entrance to a temple of education: it speaks of classical wisdom and the rule of reason. We boys weren't allowed to go in that way, of course.

Today, 40 years since I last saw the school, we step in through Latrobe's columned porch as though entitled. Nothing can touch us: we're parents. Ruth, my wife, grips my hand. A friend who works in post-traumatic stress disorder warned us, quite gravely, of the risks when people visit scenes of past troubles; of hyper-arousal – sweats, nausea, high heart-rate. Or the opposite, hypo-arousal: a state of lethargy, a feeling of unreality. But I'm fine. Pulse steady. People hurt you, not places.

There were no ghosts, no shocks as we toured the corridors and classrooms. I have not been looking forward to the smell. I could summon the brew: disinfectant, boy sweat, meat stew, chalk dust. An incense of misery. But it is gone. There is no chalk these days.

It is the details from other senses that clamour. The give of a floorboard in a corridor, the sunlight through a window, the shape of a wooden refectory bench, an echo of children's voices. We enter a cosy girls' dormitory where the low black beams were, suddenly, shockingly familiar. And the brick fireplace. This used to be headmaster "Billy" Williamson's study. I'd scrutinised those bricks, the way they sat upon each other, many times over those five years. Waiting for his flap-jowled face to stop shouting and get to business: detail the punishment or the beating.

Just down the corridor, two worn wooden steps led to the tiny dormitory where I slept in my first term at the school. I and the other eight-year-olds would turn our faces into our mattresses, pull pillows over our heads. If you wept out loud, the 10-year-old dormitory captain and his deputy threatened to whip you with a belt. That was their prerogative, they told us on the first night, a few hours after our mothers had extracted promises from them to look after their little ones.

Writer Alex Renton with his dog Ziggy on Calton Hill, Edinburgh. Photograph: Robert Perry for the Observer

The last seems such a cliché of boarding school life – surely the tearful mummy pleading with the bullies is in Tom Brown's School Days? Or a Michael Palin sketch? – I wonder if I've made it up. The memories are blurred. I'm shocked how few of them there are. And telling and retelling the few stories that stand out in bright light carries risks – they gather accretions. Now when I meet men who were at the school I tend to check detail obsessively – He was called what? That happened when? – as if without reaffirmation what was real might slip into the darkness. Old Ashdownians sometimes tell me things that make my jaw drop.

But I do know that after the half-term break that first autumn we came back to a terrifying dressing-down, delivered under those low beams in the headmaster's study. One of us new boys – I still don't know who – had complained about the regime in Dormitory V to his parents. This was the cardinal sin. What happened in school stayed in school. Billy punished us all. We didn't tell tales again.

Some of the key locations have shrunk absurdly small: the brick chapel where Billy gripped the Bible and harangued us with the backing of his three trustiest prefects: Jesus, the Holy Ghost and God. Just as tiny now is the assembly room where, daily, 120 boys aged seven to 13 were ranked on wooden benches. Here the diatribes, the mass punishments and public humiliations happened. This was where he would detail who had cried under the cane the previous night: "Jones and Smith took it like gentlemen. But Renton blubbed like a baby."

That was then. Now the site is the "play-room", with a cushioned chill-out area adjoining. The larky 12-year-olds playing pool round a table seem to take up half the space. In the corridor I find a familiar picture, a print of the Pietro Annigoni portrait of the Queen, done after her coronation. She is young, beautiful and brave. I remember I used to watch her during assembly. I would wonder what she would do if she knew how unjustly we, her young subjects, were being treated. I'd will her to descend, glorious like the first Queen Elizabeth, and order Billy and his staff to the Tower. Or, like Boudica, ride down on the teachers and the prefects, slashing them to bits with the spinning swords on her chariot wheels.

Alma mater: Alex Renton outside Ashdown House.

The school has prospered since, as has the whole industry: now there are 22 full-time teaching staff. In my time there were only 10 or 12, some of them just graduated, and I wonder how many of them had any qualification at all. There's a new teaching block, a purpose-built canteen, a swimming pool and a kindergarten.

Lost in this warren is the classroom where, one afternoon when I was nine or 10, a hated and violent young teacher I will call Mr X slipped his hand into my corduroy shorts and tugged at my penis. This was a known hazard – in return Mr X gave you a Rowntree's fruit gum. Mine was a green one, nobody's favourite. Is this a memory I can trust? No doubt. I can feel my face against the rough tweed of his jacket, scratchy.

As the visit goes on, corridor after corridor, a sadness grows in my chest. Afterwards, utter exhaustion. I'm very glad, though, to see these rooms now full of light and character.

Especially that. Where our walls were bare and the only softness the identical candlewick bedspreads, now there are teddy bears and family photos; posters of ponies and Chelsea footballers. Peering into classrooms, the children are lively – unnaturally polite, compared with the ones at my daughter's state primary – but no one looks unhappy. As if they would. I realised I'd sort of expected that. Little rooms full of children with faces like The Scream.

After the tour, there are coffee and biscuits – we've come posing as prospective parents – with the headmaster and his wife, a couple in their 40s. They seem kind and practical. We chat about how boarding schools have changed and who from my days stays in touch. Who sends their own children to Ashdown. This stiff conversation is interrupted by a dazed little child who has brought a letter to be sent to his parents.

The headmaster calls him "my dear boy": when the child stammers what he wants and leaves, the headmaster explains a little, adding that winter is a bad time of year in a new school. We make sympathetic faces. I say that if my daughter comes to the school, she would like not to board immediately. The headmaster nods. That's fine. Weekly boarding is good, though an initial period of no contact with parents is for the best. One of the boarders, he tells us, is just six years old. That's been fine, too. His wife nods. At Ashdown now there is, the brochure reassures you, a "warm, kind and trusting home-from-home environment". No hugs, though.

New boy: Viscount Linley, then aged 7, is left in the care of 'Billy' Williamson in 1969. There is no suggestion that Viscount Linley was abused at Ashdown. Photograph: UPI Photo / eyevine

The little boy's letter to Mummy, the scrawled envelope barely legible, lies between us on the coffee table among the porcelain like something raw. I remember how the teachers would inspect our letters home, and how we were punished if found to have complained to our parents. In a school of endless rules, offences against omerta were perhaps the most seriously policed of all. Of the platitudes from the current headmaster, only one impresses me: the school likes to encourage "independent thought". That is a change. This is a very different place altogether.

The last time I set foot in the school was the day I left, aged 13. In the 40 years since, Ashdown has loomed large – it was, as they say, "the making of me", for better and for worse. But I had not thought of revisiting. Except once, aged 15. Then a group of boys at Eton and I had discussed whether we might charter a mini-bus and visit the grave of the newly deceased Billy Williamson, to dance on it. It never happened. But the thought was good.

With the headmaster's death, Ashdown House and its demons began to fade. A young teacher, one of the decent ones, became headmaster, introduced girls and abolished the cane. The stories of baroque cruelty and insane adult behaviour became jokes, used to bond with people who'd been through the system at other schools. There was a time when the stories were fun to use to horrify girls, and another when they could serve to excuse your own emotional screw-ups. They were war stories: they made us feel special.

Sympathy in the wider world was limited. It still is. We were toffs whose parents had paid for the luxury of having their children abused – we were hardly the survivors of the care homes of north Wales or Catholic church vestries. We were not noisy: we kept calm and carried on, as trained. Some of us would later untangle the memories in therapy.

There has clearly been some demand for that. By the 1990s this odd corner of the British ruling class's mechanisms had become a subject of academic study and the grounds of psychiatric careers. Now Boarding School Syndrome has a symptomology, "survivors' groups" and it's a thriving area for counsellors and psychotherapists. Private, of course. Money buys you entry: a friend who works in psychological trauma in the NHS says she's never come across this particular field.

The story of the British boarding school and its experiences have been widely written, as one might expect. Reading the raw accounts in the recent press and on abuse survivors' web fora I find myself flinching a little at how nicely turned some of the accounts are: like the lesser poets of the First World War, the emotional effects are just a touch self-conscious, the result, probably, of too much Wordsworth and Keats in the Fourth Form. Strange, to educate people to go out and be cannon fodder, but also to describe the experience like a Romantic.

Most of the professional memoirists seem to have ended up accommodating their boarding school experience: "Hell, but it made me the man/woman I am." Some who loathed their school days end up endorsing the system. Among these are Winston Churchill and Richard Dawkins. In his new autobiography, the scientist reckons the "mild paedophilia" he encountered was of its time and thus acceptable. 'I am very conscious that you can't condemn people of an earlier era by the standards of ours," he has said.

Far from condemning, others acknowledge they owe their careers to the emotional catastrophe of their education. Generations of male, middle-class British comedians' currency is the dry, dark humour that comes straight from the coping mechanisms of upper-class suffering. The other day, Eddie Izzard spoke of how he was sent to board at seven, shortly after his mother' death. He "cried relentlessly for a year… My housemaster would help me along with beatings when he could fit them in."

That brand of wry fatalism is characteristic. My wife Ruth did not go to boarding school. She says she cannot stand it when people who did talk through their experiences in "endless infantile grim jokes". But modern British culture has swallowed the boarding school story and digested it, caring not very much. It was an anachronism, a hangover of the imperial age, and in the 1990s, it looked as though the boarding schools were dying out. Numbers of boarders were collapsing.

Then new money and changing fashion brought about a curious revival. Another generation of the rich started sending their children away again. Once again, the little ones demanded it, they said, because of the books they had been reading. Only this time, the propaganda wasn't Enid Blyton's Malory Towers or Anthony Buckeridge's Jennings but JK Rowling (who did not go to boarding school and doesn't send her own children to one). Perhaps Harry Potter revived the English boarding school: numbers of boarding children have stayed stable since 2000 and through the recession. There's about 70,000 of them. As far as I can work out, around 4,000 of those are 10 or younger.

Now, of course, the country has had four years of its own experience of the effects of boarding school. The majority of the 2010 coalition cabinet were privately educated, most of them as boarders. Boris Johnson went to my schools – Ashdown and then Eton, and Andrew Mitchell, the former chief whip, went to Ashdown before going to Rugby. Of course, there's never been a government, even a Labour one, in which privately educated people were not among the major players. But, as critics like to point out, this clutch of male ruling politicians embodies the grand Victorian public school virtues – or failings – more than most: suppression of emotion, devotion to the team, distrust of women and minimal empathy for the weak and ordinary.

And so it is interesting that so many senior politicians in government went to boarding schools, places that, by definition, practise on young children the techniques of "attachment fracture" – a psychiatrist's phrase – that are key to removing early emotional ties and building esprit de corps. Of those politicians quite a few – including the chancellor, the prime minister and deputy prime minister, the Mayor of London and the Attorney General – were at private schools where teachers from their era have been accused or convicted of sexual abuse. The coalition is quite an advert for the old way.

For two decades there has been talk of an enormous abuse scandal brewing behind the facade of the 20th-century British private education system. Last December the story sprang to life. A long-running case against Peter Wright, the 83-year-old former headmaster of a school in Buckinghamshire called Caldicott, came to an end. Wright was found guilty of 12 counts of sexual abuse: one of the piquant details was that Nick Clegg had been joint head-boy at the school; his colleague, the other head boy, was one of the principal witnesses against Wright.

He had first been charged 10 years' earlier. A judge threw out the case because the offences were "historical". When the trial that finally went ahead in 2013, some of the 1960s allegations were not admitted. (Wright has now been jailed for eight years, having been found guilty of 10 indecent assaults and two counts of gross indecency, between 1959 and 1970). At least 30 pupils were involved, according to one newspaper. Five other teachers were implicated, one of whom threw himself under a tube train before the trial. One Caldicott teacher, sacked in 1972, went on to teach and abuse at the Harrodian School: the law did not catch up with him until 2003. Another abused children at Caldicott and a school in Shropshire in the 1970s and 1980s and was finally jailed this February. There is, not for the first time, talk of a "ring" of paedophile teachers operating in the 1970s in prep schools and public schools.

With the Wright case done, the principle that ancient allegations could be successfully prosecuted was established. It was certain more would follow. In mid-January the Times's hard-working reporter of child abuse scandals, Andrew Norfolk, wrote of a "surge in criminal prosecutions" and named 130 private schools who have been or are now subject to similar allegations. That was 5% of all in the UK. It included 50 in the independent schools' premier league, the Headmaster's Conference. Twenty of them were feeders to Eton. In the same story, Norfolk pointed out that teachers from 62 different private schools had been convicted of sex crimes against children in the past 20 years – 18 convictions since 2012. Norfolk called it "stealing their childhoods".

This all passed me by, until, last December, a story appeared in the Times naming Ashdown House under the headline "Prep school faces claims of physical and sexual abuse." The Daily Mail was gripped, because Ashdown in the 70s had not just been the school of Boris Johnson but also the actor Damian Lewis and the Queen's nephew David Linley. Linley was my contemporary – in the Mail he was quoted reminiscing without any affection about the "Dickensian" school. He remembered Billy Williamson caning his whole class for one child's "fairly petty" offence.

The old man did like a big gesture. I remember him – huge, red-faced, ranting – threatening to cane the entire school unless a boy owned up to some particularly infuriating crime. I think it was a broken window. An older Ashdownian told me Billy had actually once attempted that feat – giving 120 boys "six of the best" in one afternoon. Despite being a keen golfer with a good swing, which he practised on the Royal Ashdown course most weekends, Williamson ran out of steam. He gave up having got through hardly half of them.

But the new allegations were about more than caning, which was a legal practice in independent schools in England and Wales until 1999. (About the same time, the independent schools for the first time became subject to full state inspection.) The Mail story talked of "horrific attacks" by two teachers in the early and mid-70s. The paper had been leaked details of a campaign for compensation started by former pupils, in part because they were infuriated that a complaint about abuse made in 2003 had been brushed aside. This group had been taken on as no-win-no-fee clients by the solicitors representing some victims of Jimmy Savile.

Through friends who had also been at Ashdown, I got more detail. The complaints, from a group a few years younger than me, were indeed horrific: sexual acts much more intrusive than Mr X's pathetic bribe and fumblings. There was talk of blackmail and predatory older pupils encouraged by abusive teachers. That chimed with my adult perception of the bizarre sexualisation of life at Ashdown, especially the system of reward and discipline. At least two children I knew who had been given authority over younger kids used it to force sexual contact – Williamson seemed to have turned a blind eye. There was talk of connections with house masters at Eton, to which Ashdown fed pupils. There had been at least one suicide. There were more teachers in the complaint.

The Mail's story quoted from an email that the former Ashdown pupils were passing around: "The abuse that occurred continues to have a dramatic effect on a number of lives, with regards to ongoing relationships, career and treatment for dealing with the psychological damage it has caused. Therefore we are seeking compensation with regards to a civil case against the school."

I was very shocked when I read this – more than I could easily understand. Ashdown had had a dramatic effect on my life, too, but I had thought it was done. I didn't feel any need for revenge or compensation for what had happened 40 years ago. I didn't think I needed catharsis, either – I had long ago let light onto what I experienced at Ashdown. I was open about it with lovers, friends and family. I was, as much as I could hope to be, at peace.

But I realised I owed support to others who might need revenge, relief from the history – or money. There was another pressing need. The abusers had all been young men, if my memory could be trusted. They could still be teaching. Why hadn't I acted on Mr X years ago? I could not explain that.

The first thing I did, though, was to email the Daily Mail story to my parents. This was not revenge. That my career at Ashdown was a mistake that they deeply regretted was something we had established a long time ago. If there was anything to forgive, I had forgiven it. I know they loved me. They were victims of a terrible fraud.

Besides, the sexual abuses were, in my version of the story, just detail: the real narrative was of five years of deliberate crushing of our individuality, the suppression of emotional freedom. Sexual bullying seemed just a part of the violence and cruelty that was the basic currency of the school and hundreds like it; the tools with which it squashed our little forms into the mould. Out of it would come upper-class Englishmen and women – ready to go and run an Empire or, at least, take charge of lesser mortals with normal feelings.

So went my thinking. Nothing unfamiliar: it has been said by British liberals from George Orwell onwards. Psychiatrists I have spoken to agree that, yes, while sexual and physical abuse is the headline grabber (and what makes criminal cases), real damage is done to children and adults by long-term psychological abuse. A child may recover from a blow, but not from the withdrawal of love and the denial of safety – the "complex trauma" child psychologists talk of. Comfy with my understanding, I was someone who had dealt with his schooldays.

But then I got an email back from my mother. What she wrote stripped away my reading and intellectualising, like so many useless bandages. She said I had told her about Mr X.

Then I cried. Because that summoned a picture: a small boy, nervous, excited in his new clothes and tie, ready to drive to Ashdown House on a September morning in 1969. My little brother and sisters gathered round to wave me off. A few weeks before I'd sat up late to watch the astronauts land on the moon on the TV. I wanted to be an adventurer, too. This journey seemed like the beginning. I was as brave and trusting as only the innocent can be. I never really trusted an adult again, not until I was one myself.

My mother's email upended my 52-year-old's view of my Ashdown self. I'd thought that September day was the last of my bravery. That I had been crushed, totally. In a privately published book of appreciation given to Billy Williamson for some anniversary shortly before his death in 1976, there is a selection of some pupils' prize-winning work. It includes a story I wrote when I was 12.

It's a plain account of a self-centred little boy who is given a rabbit for his birthday. The novelty wears off, the boy, careless and cruel, fails to clean the hutch or feed the rabbit. His mother warns him to look after it better; he punishes the rabbit by throwing mud and stones at it. When eventually it dies, he weeps as he buries it. But then a few days later, when a fox digs up the rabbit's corpse, he doesn't notice. "Nor were the flowers on its grave ever renewed," is the last phrase.

That story of the selfish little boy and the all-knowing adults has long seemed to me as good a totem as any to show that the school had done the job it was paid for. Ashdown had broken me, as you do when you train an animal, and then drilled me until I was a suitable citizen. But my mother's revelation showed I had kicked back. In fact, I had broken the most important of all of Billy's rules. I had told tales out of school. l specifically said a teacher was touching me in a way I didn't like and that I "hated" him. And she had gone straight to the school to raise hell with the headmaster's wife.

That may explain Mr X's disappearance shortly after (to teach at another school, according to the Ashdown School Bulletin of that year). It may explain the way the headmaster targeted me in the following years, singling me out in front of the school as a fraud, a failure and a perpetrator of "filthy behaviour". But it gave me a new vision of the brave little boy who wouldn't be cowed by Billy Williamson: the boy who spoke out.

And so I thought of those others, today and in the past, at Ashdown and all the other schools who wanted to speak up. I thought of the children in council care homes, in borstals and mental institutions, who over the years were left in thrall to adults without protection. I thought of the ruined marriages, the let-down kids, the suicides, the stunted and miserable lives – the great swathe of collateral damage that psychological trauma leaves. I thought of all the kids taken from their homes too early and thus denied, as the writer David Thomas once put it, the chance to love.

I thought particularly of the 45,000 under-10s in the UK who are in local authority care today. Of the 2,000 or so kids nine or younger – too young, according to any child psychologist – whose parents are now sending them to boarding school. For convenience, or notions of status, or just because they did not love them enough, to taking a mad gamble with their children's emotional health, with their lives. And I thought of all the head-teachers who have protested that schools risk being closed by the legal actions, that that was then and this is now, that the abuses of the old boarding schools could not possibly happen today. I decided to go and see Ashdown. And I decided to talk to the police.

If you have a story you would like to share anonymously, email Alex Renton (alex.renton@observer.co.uk) in total confidence. If you have been affected by the issues in this story, call the National Association for People Abused in Childhood free on 0800 085 3330 from landlines, 3, EE, Vodafone and Virgin mobile phones, or 0808 801 0331 from O2, EE and Vodafone mobile phones.