It’s five in the afternoon at the Forest school, a boys’ comprehensive near Wokingham, Berkshire, and most of the staff and students have gone for the day. But headteacher Mary Sandell is still in her office, as she is every day. The room, with its slatted blinds and motivational messages – “Make your mark on the world”; “Keep your promises” – is the one she entered as a newly appointed head three years ago. Her buzzword then was “‘fizz: I want to go into classrooms and feel fizz. I want children to feel energy and passion.”

Now all she feels is battle-weary. A combination of budget cuts and underfunding means Sandell had to axe food technology GCSE (along with the food technology teacher) from the curriculum last year, because the school “simply couldn’t afford it”. The subject had been on offer since the school opened nearly 60 years ago. Now she has to make further announcements: next year there will be no music, French or Spanish A-levels (no languages, at all, in fact, beyond GCSE), because they are no longer “economically viable”.

Nearby, in the Winnersh Triangle business park, an expanding hub of tech companies, the future looks shiny. Not so for the Forest school. “Here we are in 2017, and we’re going backwards in terms of choice, not forwards,” Sandell says. She sits brooding at her desk and considers the future. She could put up with the shabby paintwork, the rundown toilet blocks and substandard tennis courts (only two of the five have nets), but not with the fact that there are only 45 geography textbooks for the 168 pupils doing geography GCSE. There is an e-copy, but not all of the boys have a fast internet connection at home.

I love what I do, but there comes a point where resignation is the only option

On 31 August, after 29 years and 43 days first as a teacher, then a deputy, then a head, Sandell will be standing down in protest at what she sees as a crisis in education. “We are short-changing our children, and by that we are short-changing the nation,” she says.

She is not the only one who feels this way. Last month, Alex and Peter Foggo, the head and deputy head, respectively, of a primary school in Hampshire, announced that they would be resigning after 25 years in education. “The last year has seen things just get harder and harder, as more and greater challenges have come to undermine our core beliefs in education,” Peter wrote recently. Last September, Jo Garton resigned as head of Bridlewood primary in Swindon, after eight years. Concerns about school funding levels, changes to the curriculum and new assessment methods had led her to the conclusion that children were being “used as guinea pigs by politicians, yet again”, she wrote in her resignation letter to parents. “This is unforgivable.”

“I love what I do, but there comes a point where resignation is the only option,” Richard Slade, head of Plumcroft primary school in Greenwich, south-east London has said. His school faces a £400,000 cut in funding by 2019-20. “If I’m going to deliver those cuts, standards are going to be unsafe.”

He told delegates at a recent Westminster Education Forum that “a lot” of headteachers have told him they are considering resigning, after asking why they would want to “oversee the decimation of our schools”. Last month, more than 500 headteachers signed an open letter to Theresa May, demanding that she abandon proposed education cuts, which they said represent up to £3bn in real terms.

“It’s a Headteacher Spring,” says a spokesperson for the school leaders’ union, NAHT. “An uprising. Lots are writing to MPs, governing bodies, taking an unusual interest in campaigning.”

In the 17,000 primary schools and 3,000 secondary schools across England, discontent runs deep. Research by the NAHT shows that 72% of heads say their budgets will be untenable by 2019/20; 18% of school leaders say they are already in deficit. But the pressures are not just financial. There is also a new national curriculum in local authority schools in England, for pupils aged five to 16, and new Sats tests. The combination of external auditors (Ofsted), government targets and league tables has massively increased the stress. Work now routinely entails detailing aims and objectives, analysing data, and the anxiety of being subjected to ubiquitous scrutiny. The micromanaging by the Labour government under Tony Blair – literacy strategies, numeracy strategies – has shifted under the Conservatives to a focus on structure, with the championing of academies, free schools and now grammar schools.

In recent years, Ofsted has increased the pressure, bringing in “no-notice inspections”. A school “doing badly” can result in the head being dismissed. “Heads now join the likes of football club managers, whose job can often rest on the result of one major match,” one headteacher wrote.

One of the effects is that people are being deterred from entering the profession. Sometimes there are no applicants for a headteacher vacancy. A 2016 report by Teach First and another charity says that “by 2022 England could be in need of up to 19,000 school leaders”. According to Russell Hobby, general secretary of NAHT, the profession is headed towards a crisis: “School budgets are at breaking point, and so are many headteachers.”

***

The Forest school is a collection of unremarkable buildings in a residential area near Winnersh station. There are 1,200 pupils: boys-only up to year 11, and then mixed in years 12 and 13. The atmosphere in the corridors is energetic and enthusiastic, with a lot of optimism focused on the school’s sporting achievements. “We win every sporting cup in the area,” says Sandell, who is 59, divorced and lives in Windsor, Berkshire. Marcus Willis, the unsung tennis player who played Roger Federer at Wimbledon last year, is a former pupil. “He used to play on our courts,” she says. “Now, the surface is not safe to play tennis on.”

Sandell, the oldest of four, grew up on a council estate in Swindon. Her father worked in a factory, her mother was a school cook. She went to a “fairly crummy comp” in Swindon and from there to Manchester University, where she studied geography. She went into teaching because, “I love kids and I love geography, so it made sense.” She joined the Forest school, her sixth school, as deputy head in 2011.

She is evangelical about teaching (and unusually for a head, still teaches). “When I’m in front of a class, my ambition is to inspire. Switching the child on in that way is what it’s all about,” she says.

But then she returns to practicalities. She points out that Wokingham is one of the lowest-funded boroughs for education in the country. The current funding system is complex and determined by each local authority, resulting in some areas receiving more money than others. In 2016/17, the Forest school, for example, received around £4,166.51 per pupil per year, compared with £6,982.07 per pupil in Tower Hamlets, in east London, according to a report by the Education Funding Agency.

There is the promise of more money for the Forest school under the new national funding formula, due to be introduced in 2018-2019, which is designed to iron out regional variations; but the whole operation will still be underfunded, because government spending on schools has not kept pace with rising prices and rising pupil numbers. The Institute for Fiscal Studies predicted that spending per pupil was likely to fall by around 8% in the five years from 2014-15.

All of which means the Forest school’s financial troubles run deep. Even becoming an academy, which was meant to represent financial freedom from the local authority, didn’t help. “There was a short-term benefit: a small cash sum given at the time as an incentive,” Sandell says. “But we would have been better off staying with the local authority, because then you’ve got somewhere to turn.”

Even less helpful is the fact that employers now have to contribute more to employees’ NI and pension contributions. “That is costing us £100,000 more a year – that is, three young teachers, money that has been diverted away from children, resources and learning.”

Sandell sighs heavily, exasperated. “Last year, we didn’t replace seven teachers; this year, we won’t replace three.” Drama was set to join languages, music and further maths as a defunct A-level subject, but the head of drama stepped in. “She said she would teach the boys in her own time, after school, because she knew how passionate they were. We’ve managed to prevent that from happening, but she’s going to teach year 12 and year 13 together, which isn’t ideal.”

Earlier this year, Sandell reached breaking point. “I am a capable woman, I could stay, and continue to make those cuts and to teach with fewer textbooks and so on. But I don’t want to, and I don’t think it’s right. I feel somebody has to say, ‘This isn’t good enough.’ And that’s what I’ve done.”

Her plan is to carry on supporting the Fair Funding For All Schools campaign. In the meantime, she may move to Brighton and enjoy some of those hobbies she has let fall by the wayside: reading, crosswords, gardening, meeting friends. “Friends joke that I’ll be doing yoga on mountains,” she says. “I used to go to the gym about three times a week for 30 years. I haven’t done it at all since I’ve been head.”

Before I leave, I write down a list of the things she says the school needs: toilet blocks, textbooks, computers, reinstated subjects, teachers. Quite a challenge for whoever comes next.

Alex and Peter Foggo, head and deputy head of a primary school in Hampshire, are stepping down after 25 years. Photograph: Kieran Doherty

It was Kenneth Baker, then education secretary, who spearheaded the launch of the first national curriculum in 1988. And it is Baker who has criticised the most recent national curriculum (2014) for being “Victorian”. In the intervening years, the curriculum has been slimmed down (1994), overhauled (2000) and earmarked for reform (2007), a plan subsequently abandoned in a change of government (2010).

“The consistent thing is change,” Peter Foggo tells me. “New secretaries of state are appointed and they want to make their mark. You are constantly having to change focus and accommodate whatever the latest thing is, knowing in the back of your mind that in time that, too, will go. The curriculum we are currently being forced to deliver, I have no doubt, will be gone and replaced with something else in five years’ time.”

Peter, 56, and Alex Foggo, 51, live in a village in Hampshire. Peter has a 28-year-old daughter from a previous marriage. They have been at Longparish school, a Church of England primary, for 11 years: Alex as head, Peter as senior leader and then, last year, deputy head. They have had to accept decreases in funding and cuts to staff, to comply with the “gradual creep” of accountability, recently intensified by such government initiatives as “school-level progress measures”, and to exist in a state of constant readiness for a short-notice Ofsted inspection.

“It’s extraordinarily stressful,” Peter says. “You sit and wait for the phone call, knowing that it comes between noon and 1pm, and then, after one, you breathe a sigh of relief.” Ofsted will not call a school on a Friday. “After you get past one on a Thursday, you can rest easy until the following Monday.”

We go through a child’s writing. Is there a conjunction? Tick. An extended noun phrase? Tick. Do we actually read it? No

But for the Foggos, the real problem is the new national curriculum. “What makes it narrow and dull isn’t the content, because there is lots of flexibility in terms of what you can teach – it is the high-stakes test at the end of it,” Peter explains. The school is assessed on the results of key stage 2 Sats, which children take when they’re around 11, and poor results can trigger an Ofsted inspection. So the system, which is concerned with assuring quality, actually hinders innovation and creativity. Especially because there is now, in Peter’s view, a “bizarre and almost completely useless” new focus on grammar and spelling.

Children are expected to know the past-progressive tense, for example, and get credit for using exclamation marks only in sentences beginning with “what” or “how”. “What big teeth you have, Grandma!” says Red Riding Hood, is correct. “Wow! Big teeth!” says Red Riding Hood, is wrong.

“What we do now is sit down and go through a child’s piece of writing,” Peter says. “Is there a conjunction? Tick. Is there an extended noun phrase? Tick. Do we actually read what the child has written? No. It could be complete nonsense.”

“Last year was the first time ever we were having to teach to the test,” Alex says. “And that was just horrible.”

“My fear is children are going to be switched off from learning,” Peter says. Especially dyslexic children, who are hit particularly hard by the new focus on the technical side of writing, rather than the creative joy. Some children were “utterly crushed” when last year’s results were released.

The couple were walking in the Lake District during the Easter holidays when they decided to resign. “When you are asked to do more and more things that are just so fundamentally opposite to what you believe, there comes a time when you’ve got to say enough is enough,” Alex says. “I went into teaching to make learning irresistible, to make children want to learn. There are only so many ways you can make subordinate clauses joyful.”

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Sam Offord, 49, headteacher at Birchfields primary school in Fallowfield, a suburb of south-east Manchester, sits in her office surrounded by rabbits: plush toy rabbits, rabbit-patterned mugs and cushions, rabbit magnets, rabbit-themed storybooks. On the wall behind her desk is a gallery of rabbit photographs: Whizz and Rosie belong to the school; Bernard belongs to Offord. “I bring Bernard to school sometimes. He’s got a pushchair and all the children stroke him, and I put him on a lead and we go out on to the fields.” She smiles: “Children love that.”

Offord, who grew up in Doncaster, is from a family of teachers; she has been a teacher since she was 22, and a headteacher since 2001, the last 10 years at Birchfields. She lives in Cheadle Hulme with her long-term partner, a deputy head, and is childless by choice. “In effect, the school is my child. I love my job, and I know people do both, but I felt I couldn’t take on the role of mother and give as much as I give to the school.”

She has worked in some of the more disadvantaged areas of the country, but even by the standards with which she is familiar, Birchfields is exceptional. “We are 98% ethnic minority children, mainly from Bangladesh and Pakistan. We have 25 different languages, 40% of children are on free school meals.” Offord is responsible for 98 staff and 720 children.

“I really love my job,” she says. As head, she has “influence”, enjoys giving children experiences they wouldn’t normally have: trips to the Merseyside Maritime Museum, planting boxes in the school gardens. She likes seeing her staff develop. “When it all works, it’s fantastic.”

But in May 2012 her career nearly ended with a phone call. It was a senior quality assurance adviser for Manchester local authority. “She said my results were poor and an academy broker [a consultant employed by the Department for Education] was coming to look at the school.” Offord was facing what’s known as a “forced academisation”, where, in certain circumstances, the regional school commissioner can order a school to become an academy without a governing body’s consent.

“I was shell-shocked,” Offord says. Results were poor, she admits, but she had a strategy. When she arrived at the school in 2007, she decided to invest in the younger children, get the basics in at the start, and watch that year group flourish as they moved through the school.

Sam Offord, head of a Manchester primary, says her budget has ‘gone off a cliff’. Photograph: Stephen Burke for the Guardian

For Offord, poor results from 2007 to 2011 were to be expected; success would come when her invested-in children sat their Sats in 2012. “We knew they were going to be good, and then they were going to be good for ever after. We’d had five years of working hard, improvements being put in, the vision, the strategy – it was coming through.”

She said all this to the adviser on the phone, and was told, “That’s irrelevant now. They’re coming.” She said the same to the broker, when she came to the school. “She said, ‘Yes, that’s fine, but you’re still going to have to become an academy.’”

Offord focused her fightback on the results. “Talk about pressure,” she says. But they were as she’d hoped: above the national average. “It was complete, utter relief.” She emailed the broker. “She said, ‘While you’ve got these results, the school is not a priority to become a sponsored academy at this time.’”

“Eighteen months is the average time a head stays after a school has become an academy,” Offord says. “Heads used to lose their jobs if they were fiddling the books or doing something inappropriate to children; now they are losing their jobs because their face doesn’t fit.”

Twelve of that first cohort of children went on to selective independent schools in Manchester, most of them with a bursary. “So there is that success, but there is also the success of children who came here with nothing and have managed to integrate and become model students.”

But now, of course, the outcome for Birchfields is a budget “that’s gone off a cliff”, meaning staff redundancies and, for Offord, new stresses. She is determined to stick with the job. “You don’t say, ‘I can’t do this’, you say, ‘I can’t do this yet.’” Or at least that’s what growth mindset, the positive-thinking theory she is trying to adhere to, suggests. But she is worried about staff morale, which is low.

There is another manifestation of stress: illness is endemic. Offord was unwell over the summer of 2012. “I think the academy thing just took it out of me,” she says. Alex Foggo has insomnia. “Certainly I don’t sleep at night any more – or very little. Or you wake in the middle of the night and have got school on your mind.” Mary Sandell feared for her mental wellbeing had she continued. Jo Garton, 52, had intended to work until April after she resigned, but was signed off sick six months before, diagnosed with stress and anxiety. “Headteachers have to be social workers as well as masters of grammar now,” she says, explaining that funding for social care and child and mental health services has also been cut. This places a huge burden on headteachers, because they are often unable to call upon a functioning and efficient support system to follow up on cases of suspected abuse.

“We are the welfare state in our community,” says Garton, who is married with two adult children. She had a hands-on approach to pastoral care at her school in Swindon. She wanted to keep children in school when she feared for their safety. “I’d phone social workers and say, ‘I am going to stay with these children until you do.’ And I wasn’t the only headteacher who did that.”

But a depleted and inconsistent system was often unable to respond. “I’d wake at two in the morning, worried about children,” she says. “And I couldn’t keep doing that for another 10 years.” She is now doing consultancy in schools and hosting an educational programme on her local radio station, and says she feels “massively better”. “Stress manifests itself physically. I had IBS and as soon as I stopped working, it stopped. I’ve been with other headteachers who bring Gaviscon out of the bag and glug it back, because they’ve got acid stomachs, ulcers, whatever.”

But for Garton, as for the other headteachers, it is the pupils who are the real concern. “A lot of people have stressful jobs,” Alex Foggo says. “It’s more the effect you see it’s having on the children. The pressure they place themselves under, even though you try not to let them do that. The thought of the 11-plus coming back, where they label themselves as failures, is just awful.”

While education continues to be a battleground in the run-up to the election, with each party offering different strategies and funding proposals, the children need something now. “They have,” Alex says, “only one shot at education.”