Yo Hirano (14) was cheerful and well-adjusted when he transferred to a new school in Tsukui, near Tokyo, in April 1994. Within three months, he had hanged himself in his bedroom wardrobe, driven to suicide by a gang of schoolyard bullies and unprotected by teachers who looked the other way.

The story of Yo is not an unfamiliar one. Over the last 25 years, more than 5,000 schoolchildren in Japan have taken their lives, how many due to bullying is not known. But Yo's case became different. His parents sued and recently won a landmark judgment that held responsible not only the local government but also the bullies. In an unprecedented ruling, the court awarded 41 million yen (£300,000) in compensation, of which Yo's tormentors, six boys and three girls, were ordered to pay just over £10,000 apiece and the local and prefectural governments the remainder.

Yo was an easy kid to pick on. One of the smallest boys in his class, he had moved mid-term to Nakano Junior High from a school 20 miles away, making him an outsider in a land of groups. He was also, according to his father, tough-minded, refusing to back down or run away. The combination of difference, stubbornness and physical weakness was enough to earn him the attention of other kids in a country where "the nail that sticks up is hammered down". The famous Japanese proverb acknowledges both the value attached to conformism and the way it is sometimes achieved.

From the moment he arrived at his new school, Yo was subjected to a relentless bullying campaign that only ended the day he died. The opening salvo was a letter from a female student, which was circulated to the rest of the class for their amusement. Yo would arrive at school to find words like "idiot", and "little creep" scrawled on his books. One bully wrote "f..k off and die"; another scribbled "I've put a curse on you which can only be lifted when you die".

Students put chalk dust and thumbtacks on his seat and laughed when he sat on them. From June, the campaign escalated into physical violence, at first shoving and later punching and kicking. The day before he died, he screamed within earshot of three teachers that he couldn't stand the torment any more.

His parents knew nothing of his ordeal and the school wasn't telling. The only inkling his teachers offered was when one explained a mark on Yo's face by saying he had been pinched by the winner after losing a game of junken (rock/paper/scissors game), something that commonly befalls junken losers. It later emerged that Yo had been forced into playing the game and punched, which the teacher knew. Before the suicide shook their world, the Hiranos neatly fitted the Japanese post-war family stereotype: salaried dad, housewife mother, small house in the suburbs. Even the manner in which Yo, who had borne his ordeal in stoic silence, went about taking his life suggests a matter-of-fact ordinariness. On his final day, he was playing a computer game downstairs in their small home. "How about doing some study?" his mother said. He went upstairs. She called him a little later to come down for his dinner. She called again, went up to his room and found her son hanging from a closet rail. No drama, no warning, no suicide note. Yo's devastated and bewildered parents later became angry with what they saw as the local educational establishment's attempts at a cover-up. Bullying-related suicides attract attention from the media and show the school and the district in a bad light.

The cover-up, they believe, began from day one when school representatives came to pay their respects. Yo's teacher began to blurt out in front of the Hiranos "after all our efforts it came to this", but was stopped by the head teacher elbowing him to shut up. At his funeral, among the hundreds of mourners from his previous schools in their distinctive uniforms, only 10 students from Nakano School appeared, a turnout that crushed the parents. The school had told students not to go, afraid that prying reporters would question them about the circumstances behind Yo's death. The strategy didn't work, and the Hiranos found out about the final nightmare months of their son's life through the local newspapers. School managers and officials from the local educational council denied that there had been bullying and repeated in the press and elsewhere that the "incident" took place at home and therefore didn't involve them. According to Kimie: "They suggested that he was an odd boy, that he somehow deserved to be picked on".

INSTEAD of inquiring into the causes of their son's suicide, local educational administrators closed ranks, pushing the blame onto the family. The Hiranos felt they had no option but to take legal action.

The road to court was blocked with questions, not least of which was the morality of taking other children to court. Court cases in Japan, where people are much less likely to question officialdom, are rare, and children are strictly protected. Yo's father acknowledges mixed feelings.

"In many ways, the bullies are victims too," he says. But the Hiranos felt that the school had, by keeping them in the dark, robbed them of the chance to prevent Yo's suicide. They also wanted to make a wider statement.

"If institutions like schools stop hiding things when they go wrong, maybe we can prevent them from happening in the future," Shinya says. Their action was supported by some school parents, but it also provoked threatening calls and taunts on the street. Kimie says the abuse only made her more determined to carry on.

But it is unclear whether the landmark legal judgment holding the bullies financially responsible will make much difference. Kimie says that bullying has changed from when she was younger.

"It seems to involve all or most of the class and, for the bullied, there is nowhere to escape," she says.

Bullying, which has been a huge issue in Japan since the mid-1980s, is often carried out by groups rather than a few mavericks and, though almost never sanctioned by teachers, it is often tolerated by them. It is not just a school problem. Bullying is increasingly resorted to by companies to force unwanted workers - protected from dismissal by custom and Japanese labour law - to resign.

For the Hirano family, the legal victory brings cold comfort, and the questions remain. While both parents concede that they have become tougher and have steeled themselves for the legal appeals that will follow, Shinya says he remains "full of sadness about why Yo never told anyone".