Mum tells of son's Knox Grammar hell

A mother whose son suffered severe mental health problems after being abused by a teacher at a private Sydney school says the family still lives with the pain.

A mother has told an inquiry her family still suffers from her son's abuse at a Sydney school. (AAP)

A mother has told an inquiry how the abuse her son suffered a prestigious Sydney school means her hopes for him are now stripped down to mere survival.

Her son, ATS, started at Knox Grammar School in Wahroonga in 2002 when he was 10.

She said her son was very happy for the first term but then changed - he became quiet and withdrawn.

He refused to talk to his parents, became angry and hostile and self-harmed before being hospitalised after a serious suicide attempt.

When he was 14, ATS revealed to a psychiatrist that he had been abused by Craig Treloar, who had been targeting boys at the school for years.

Treloar was sentenced to four and a half years in jail, with two years non-parole in 2009 and is now out. He will give evidence at this royal commission hearing.

ATS's mother said her son told the psychiatrist Treloar used to invite boys to his room and offer them ginger beer.

His mother said her son would later say things to her like "'Treloar tried it on lots and lots of times, but only got it once'".

She said she took this to mean that Treloar was constantly sexually harassing her son.

When ATS was 16 he was offered a professional rugby contract with Gordon Rugby Club, but he could not through with it.

ATS, now 22, lives in Canada - he cannot bear to live in Sydney, she said.

"Words can't express the enormity of the daily pain experienced by ATS or myself and his family," she told the Royal Commission into Institutional Responses to Child Sexual Abuse.

The strain led to her marriage breakdown.

She said the experience had been horrific, terrifying and heart-wrenching.

"As a mother I have hopes and dreams for my children. My dreams for ATS were stripped own to mere survival."

Earlier another witness, Coryn Tambling, said he was abused by Treloar in 1984 when he was at Knox.

He said the teacher showed him and other boys pornographic videos one of which was a "homosexual gang bang involving teenage boys".

The teacher would reassure the boys and tell them it was OK to be homosexual, he said.

"Treloar also told me he was part of a group in Kings Cross that made these movies and he would secure me a part in a movie if I wanted."

He said he showed the boys business receipts from the Kings Cross company.