Before 18 May 2010, the Crouches would have considered themselves a happy family. Not exceptionally so, perhaps, but comfortable enough to enjoy each other's company. Arguments were rare. They laughed a lot.

On the mantelpiece in the front room of their modest home there is a photo of the four of them on holiday in the Lake District. It was taken on an overcast day and they are huddled together, in waterproofs and walking boots, smiling in readiness for the click of the camera's self-timer. Paola, a petite, fine-featured woman with dark hair and eyes, is in the middle, flanked by her two teenage children, Giulia and Dominic. At the back, arms outspread to encircle his brood, is the father, Roger, his expression one of obvious pride.

"We used to giggle all the time – we were quite loud and boisterous," says Paola, sitting on one end of the big blue sofa backed up against one wall of the room. Her daughter Giulia, now 20, sits cross-legged next to her, not meeting her mother's eye. Instead, she fiddles with a series of coloured wristbands, kept around one arm as reminders of the various music festivals she went to last summer.

"It's different now," Paola continues, glancing at her daughter. "You feel you shouldn't be happy. I'm self-conscious about it. If I'm walking the dog and it's a nice, crisp day that puts a smile on my face I worry that people look at me and think: 'Oh well, she's all right then.'"

The photo on the mantelpiece is one of the first things Paola wants me to see when I walk through her front door. She wants, I think, to show what a normal family they were then and how, on that day in May two years ago, a devastating sequence of events was set in motion that would leave them here: two women on a sofa in an empty house, not wanting to meet each other's gaze in case they start to cry.

On 18 May 2010, Dominic committed suicide by jumping off the roof of a six-storey block of flats near his school in Cheltenham. He was 15. In the note he left his family, hastily scrawled in sloping lines of black ink, he wrote: "Dear Family, I'm so so sorry for what I'm about to do. I have been bullied a lot recently and had a lot of shit made up about me that ain't true."

At the inquest, nearly six months later, it emerged that Dominic had kissed a boy on a school trip during a game of spin the bottle. A group of Year 9 students had been on a residential trip to the Forest of Dean, organised by the school – St Edward's, an independent, Catholic school. The teenagers were staying in St Briavels Castle – a 13th-century building that now operates as a youth hostel – and one mild evening had gathered in a field to play the game.

At the time, it seemed little more than an example of adolescent high jinks. But some participants recorded the game on their mobile phones and were subsequently believed to have been circulating the images among other friends. Dominic, who was dyslexic and found it difficult to formulate a quick retort, allegedly became the butt of some jokes when he returned to St Edward's after the trip. Although these jokes were described by one pupil as "no more than school banter", they appeared to be hurtful enough that Dominic left three suicide notes, all of which mentioned bullying.

When contacted, Peter Goatley, the chairman of trustees for St Edward's, stated: "Following Dom's death, the police interviewed various students, including several suggested by Mr and Mrs Crouch. They did not find evidence of Dom having been bullied, and neither did the coroner, who also heard considerable evidence about Dom's life beyond the school." Mr Goatley added that the school's position is that "bullying of any sort is not, and will not be, tolerated" and that their anti-bullying policy specifically mentions homophobic bullying and has done so for many years. According to Mr Goatley, there was "no clear evidence" as to whether something had been said to Dominic on the morning of his death that might have caused him sufficient upset to take his own life.

For Dominic's father, however, there was little doubt that his son had been the victim of homophobic bullying. In the months after his son's death, Roger threw himself into campaigning on his son's behalf. He toured the country, speaking to schools, meeting MPs and writing to local councillors about the need to adopt a "zero tolerance" approach.

It is a disconcertingly widespread problem: according to a 2007 report by Stonewall, the gay and lesbian lobbying organisation, 65% of young lesbian, gay and bisexual pupils have been the victims of bullying, as have three-quarters of young gay people attending faith schools. And even if gay pupils are not direct targets, they are "learning in an environment where homophobic language and comments are commonplace". Ninety-eight per cent of young gay people hear the phrases "that's so gay" or "you're so gay" in school – and anecdotal evidence suggests the terminology is used as freely in workplaces around the country.

A Stonewall report in 2009 found that eight in 10 secondary-school teachers and two in five primary-school teachers reported hearing homophobic insults such as "poof", "dyke", "queer" and "faggot". Ninety per cent of secondary-school teachers had witnessed children being subjected to homophobic bullying.

In a speech delivered to an education conference in the aftermath of Dominic's death, Roger Crouch speculated that the "banter" Dominic heard might have been "just a way to mock a boy who was vulnerable" and outlined a series of measures designed to ensure schools took such actions seriously. According to Giulia, the campaigning was "Dad's way of dealing with it." He was determined, says Paola, that their son's voice "should be heard" and his efforts led to him being named "Hero of the Year" at an award ceremony held by Stonewall last November.

"It was genuinely uplifting to see the way he and Paola were trying to make sense, both of what had happened and of the future," says Ben Summerskill, Stonewall's chief executive, who worked closely with the couple. "It would have been terribly easy for them just to immediately move on and not consider how they might help to change other people's lives.

"We don't know whether Dominic was gay or not, but one of the sad truths of so many similar cases is that parents tend to go into complete denial, even when there is quite significant evidence as to why their child committed suicide. In these cases, those parents are going through all the emotions of discovering their child might have been gay, as well as facing the trauma of losing a child." And yet, despite Roger's outward show of strength, he was, says Summerskill, "heartbroken – and I use that kind of language cautiously."

On 28 November 2011, just a few weeks after receiving the Stonewall award, Roger Crouch hanged himself in the garage of the family home. His emotions had been battered, his energy had been exhausted, he could go on no longer. He was 55. The funeral director who came to remove the body was the same man who had organised Dominic's burial.

When she tells me this, Paola presses her hands more tightly around her mug of coffee. "When Domi died, it was like a black box opened up inside his head," she says. "It seemed to be the beginning of the end… I don't think Roger meant to kill himself. I think he meant to stop the pain… I think he fell into a dark place and he was really, really hurting."

For Paola and Giulia, the grief of losing father, brother, husband and son is so raw it is almost a tangible presence. They describe themselves as "shell-shocked" and struggling, day-to-day, to make sense of what has happened in 21 short months.

"Time does funny things when you're grieving," says Paola. "I don't feel it at the normal rate."

"You don't notice the days so much," adds Giulia. "It all just blurs into one." She looks at me, blankly. "I couldn't even tell you what day it is today."

When talking to his family and friends about what Dominic was like, there are certain words that come up again and again. "Cheeky" is one of them. "Gentle", "kind" and "hysterically funny" are in there, too. Ty Griffiths, a friend from primary school, says: "I don't think I'd ever seen him upset. He was always finding funny things to laugh at or doing something so spontaneous we would all laugh with him." Calum Wood, who knew Dominic from the age of three, remembers him as "a small guy with a big heart" who loved climbing trees and would go barefoot wherever possible. "There used to be hand and foot marks along the walls of his hallway where he would shimmy along like a monkey," says Calum.

Giulia, his sister, who is now studying history at Leeds University, remembers that the two of them never argued. "It sounds really stupid," she says, unable to stop the tears, "but we used to play on the PlayStation this game called Crash Bandicoot and he was so good at it. He had really good taste in TV and films – he loved The Inbetweeners and Avatar. I'd just like to talk to him about stuff because he was always really enthusiastic about interesting things."

Paola remembers him asking questions about the civil rights movement and apartheid in South Africa over the dinner table where his father, who was a Cambridge history graduate, would try his hardest to answer them.

"He had questions about everything," Paola recalls. "He'd think deeply and be deeply affected by things. Part of what makes this so hard is that I was excited about what he'd become, about seeing what kind of man he'd grow into because I know it would have been something special."

Dominic had gone to school as usual on the day he died, without any suggestion there was something amiss. The evening before, he had returned from the St Briavels art trip. Both Paola and Giulia remember him being extremely cheerful when he came back, full of stories about how he had helped a friend who suffered an epileptic seizure. Victoria Widdows, a schoolmate of Dominic's who was on the trip, remembers: "He seemed his happiest. I never saw any sign of Domi seeming upset in his last few days. He was a quiet boy and sometimes liked to keep things to himself, but if I ever saw him looking sad, I would always ask him if he was all right."

Giulia, who drove him to school the next morning from the family home in the village of Gretton (a distance of about six miles), noticed nothing out of the ordinary. "We didn't talk in the car, we put on music because we both really liked the band Green Day. I dropped him off, he said he was going to some after-school thing, so he'd need to be picked up later than usual. I really don't think it was planned. I don't think he knew he was going to do it."

Whatever happened that morning after Giulia left her brother at the school gates, it led to Dominic skipping his afternoon lessons and walking to a nearby block of flats at around 1.15pm. Once inside, he went to the top floor and, despite being only 5ft 6in in height, appears to have jumped to open the ceiling hatch and gain access to the roof. At 2.22pm, he texted 999 notifying the emergency services of his location and his intention to commit suicide. He received an automated response, which read: "You texted 999. No emergency service has been alerted. You must be registered to use this service."

How would this message have affected a distressed and dyslexic 15-year-old boy? Paola does not like to think about it. "If the school had just called me and said he wasn't there," she says. "I would have called and called and called [his mobile]. I would have liked to have had that chance."

St Edward's has said that although a teacher noticed Dominic's absence, they "regret" normal school policy was not followed in this instance and that the school has "taken further steps to ensure it does not recur".

At approximately 2.50pm, Dominic ran and threw himself off the roof of the building. He landed face down about 26ft away on the driveway. Six minutes later, an ambulance was called and he was taken to Cheltenham hospital with multiple fractures of the skull, pelvis, arm, leg and facial bones and internal injuries to his lungs and liver.

The hospital called Paola using her son's mobile phone and for a moment she couldn't understand what was happening. Then the truth began to dawn. When she got to the hospital, Dominic was in a coma in intensive care. "It was like a slow-motion scene from a film," Paola says. "There were lots and lots of people looking at me and I said to them, 'Don't you dare say it, don't you dare.' I went in to see him and collapsed on the floor. Apparently we were there for five hours. I can only remember about two minutes of it."

Dominic died from his injuries shortly before eight o'clock that evening. "It shocked me more than anything I've ever experienced," says Paola. "I would never have imagined Domi would do something like that."

Giulia has thought long and hard about why her brother did what he did and why, if he was being bullied, he never once confided in his family. "I think maybe he just found it embarrassing," she says. "I never would have spoken about it if I was being bullied."

Fergus Pickles, 17, who became friends with Dominic through the local rugby club, agrees: "I think his dyslexia meant he often found it difficult to interact with people. He was always friendly but, if you didn't know him, you could sort of see why he was a kid who came in for some stick. He was easily misunderstood."

For a quiet, shy boy like Dominic, it would have been hard to admit if his contemporaries were calling him gay. According to Ben Summerskill, part of the problem in compiling wider data on this issue is that many victims do not want to speak openly about it or to confide in teachers. "That's one reason we suspect teenage suicide precipitated by homophobia is massively under-reported," he says. "Even if there is clear evidence a teenager has taken their own life because they were being bullied for being gay, coroners often don't make reference to it because they think – maybe misguidedly – it might upset the family."

In the United States, there has been a worrying spate of young suicides linked to homophobic bullying. Over a single week in September 2010, there were five deaths: 13-year-old Seth Walsh hanged himself in his California back yard; Tyler Clementi, 18, jumped off the George Washington Bridge; Asher Brown, 13, from Houston, shot himself in the head and Raymond Chase, 19, from New York, hanged himself in his university dorm room.

Shocking as these statistics are, for Giulia, Dominic's sexuality – which, at the age of 15, he was only just beginning to understand himself – is irrelevant. "I don't see why if you call someone gay, that's funny – and if you use the n-word, you get into trouble. It should be treated in the same way as racism. Domi was getting to the age where the most important thing ever was asking out girls and he was shy about that. For people to say he was gay… He was an easy target and he couldn't see beyond it. You can't when you're young; you don't have that sense of perspective to think, 'It'll be OK when I'm older.'"

Dominic's suicide was, Paola thinks, an impetuous decision borne out of an unhappiness made more intense by teenage angst. "I think it was kind of – it sounds weird – but a bit naïve. I don't even know if he thought about it too much. He was just so mortified that everyone was talking about him, laughing at him. I think it would be really difficult for anyone to feel completely singled out at school… I think it was an impulsive thing."

The barrage of intense feelings one experiences as a teenager can heighten any tiny spark of emotion and, if this is compounded by shame, an innate shyness, insecurity or a belief that one is the butt of jokes about one's sexuality, then the extremity of Dominic's reaction becomes more understandable. It also becomes more worrying for others like him, given the near-universal use of "gay" as a derisory term by today's teens.

Suran Dickson, who worked closely with Roger Crouch as the chief executive of Diversity Role Models, an organisation that seeks to prevent homophobic bullying in schools, says that the situation is "much worse than when I was younger. It's more prevalent. There are more openly gay people in the public eye and more young people coming out earlier, but what is lacking is education of their contemporaries. It hasn't caught up so there's no explaining going on from the bottom up.

"Being gay for a young person is still considered one of the worst things to be. I see it in workshops all the time. The first question I ask is: 'How would you react if one of your friends was gay? Could you carry on being friends with them?' When I asked this yesterday, 20 out of 26 kids said they couldn't be friends with a gay person, either because they believe they have Aids or because they think their friend might hit on them. In my experience, that's the same across all socio-economic backgrounds, comprehensive or independent schools."

Although there are now many gay and lesbian figures in the public eye, this has yet to be reflected in school staff numbers. "We're certainly aware of only two gay secondary-school headteachers in the UK," says Summerskill. "And that's out of 5,000 schools. That's not giving a message to teachers or pupils."

But perhaps one of the most deleterious consequences of homophobic bullying – and one that Dominic could not have foreseen – is the impact it has on entire families. For three months after her son's suicide, Paola says she couldn't get off the sofa. "I was so tired, I just couldn't get up." Her husband, who had lost his own mother at the age of 11, reacted in a different way, throwing himself into the business of raising awareness and fitting it in around his part-time job as a town clerk for Stow-on-the-Wold council. "He was almost always talking about it [the campaign]," recalls Paola. "He didn't sleep well and he'd always be at the computer early in the morning, interacting with all sorts of people."

Roger was seeing a psychiatrist at regular intervals, but refused to take medication. "I think he started to self-medicate," Paola says. "He was drinking a lot. I think he was suffering enormously. There'd be a few highs, like speaking at an education conference, but then he'd come crashing right down again."

On the night he died, Paola had gone to bed early with a book. It was only after a couple of hours, when she heard the family's Dalmatian dog whimpering outside that she went to the window and saw the light on in the garage.

"I tried to ignore it," she says. "But a part of me knew."

Is she angry?

"I don't feel angry," she says, in a quiet voice. "I feel sad. I think: 'Oh Roger, why?' It affected our whole family. I've lost my other half."

Giulia, who has been listening intently to her mother, says the house now seems too quiet: "It feels like we've lost 10 people, not two. We all had a role to play in our family and now the entire dynamic has changed."

The coffee has cooled. We have been talking for hours. Before I leave, I ask both Paola and Giulia if they will consider having their photographs taken to accompany this piece. They mull this over this for a few minutes: Paola thinks they should say yes to show they have nothing to be ashamed of; Giulia is worried that "if people see our photos, won't they just think: 'There's that weird family where everyone died'?" I leave them to sleep on it overnight.

The next day, there is a Facebook message waiting for me telling me they will send me a selection of family photographs, but that they've decided against having any new pictures taken. I can understand why. After two years of living life at the extreme edges of grief and loss, the Crouches simply want to be a normal family again.

• The Samaritans can be contacted via the Samaritans website or by phoning 08457 90 90 90 (UK).