A student went to the High Court and won the right to return to his Auckland college after being expelled for allegedly supplying drugs.

The boy, who has name suppression, went to Western Springs College and the decision, though released late last year by Justice Andrews, has been made public for the first time because his mother now wishes to urge other parents to fight for their kids if they feel they have been treated unfairly.

The student's lawyer, Simon Judd, said the rare case - about five pupils had legally challenged schools over 30 years - hinged on the school failing to tell the boy about specific allegations, thus denying him the chance to defend himself. Judd said the ruling set a precedent for all schools on how they had to handle disciplinary procedures.

Increasing numbers of children are being stood down or suspended over drugs. Ministry of Education statistics show 1017 students were stood down and 942 suspended for drugs use and substance abuse in 2007 and that had grown last year, with 1420 stood down and 1074 suspended.

The school's lawyer, Richard Harrison, had argued giving too many specifics on the allegations against the boy could have identified complainants.

However, the judge said the school could have given details of what the student was said to have done without giving names away. Then the pupil might not have been "deprived" of a proper chance to defend himself.

The school's principal, Ken Havill, said the board "welcomed the clarification provided by Justice Andrews' decision" and has since reviewed the school's practices and procedures.

The case had clarified "a grey area" and they now knew what to put in suspension reports, but he also believed the decision "affirmed" the school's ability to maintain the anonymity of informants.

According to the decision, the 17-year-old Year 12 student came under question in June last year. Parents and Auckland Zoo staff had complained about students smoking marijuana near the school.

An inquiry was carried out and someone claimed the 17-year-old was a marijuana smoker and seller.

The principal told the boy's mother he was not prepared to accept her son's blanket denials, but accepted there was no evidence he had sold it.

A compromise was the boy's mother agreed her son would be stood down for five days and go on a drug-free contract with testing and counselling.

However, the school changed its view in light of more alleged evidence and was angered the boy made a "mockumentary" video in media studies about marijuana.

No information on the alleged evidence was given to the boy or his mother. The boy was suspended and then expelled by the board - despite one complainant later saying they lied about allegations.

The school board had decided on the balance of probabilities the student had sold marijuana.

The judge found the decision to expel was invalid and gave the boy the legal right to return to school.

The mother said they had fought hard for this, but after the "emotional" and "messy" ordeal her son could not face returning there.

She was awarded about $12,000 in legal costs, which had gone on her mortgage, but this was nothing compared with the "emotional cost".

"It was worth the fight for my son's sake. It has transformed his life. He now knows he has a voice and there's fairness in the world," she said, adding that he whooped for joy and hugged her when Judd told them they had won their case.

The mum wanted to urge parents to seek advice from Youth Law about their children's rights if they encountered trouble with schools.

Meanwhile, the boy continued his studies through correspondence school and has applied to attend university next year.