A FORMER house mistress who sexually abused six boys at an exclusive school knew what she was doing was wrong, a judge found yesterday.

The woman, 41, defended 21 charges on the grounds she had a mental illness, which impaired her capacity to understand the wrongfulness of her actions.

But after a trial in the NSW District Court, Judge Anthony Garling found her guilty, saying: ''I cannot accept that she did not know what she was doing was legally and morally wrong.''

The woman, who cannot be named, was a ''house mother'' and lived on the school campus at weekends.

The court heard she initiated oral sex and sexual intercourse with the boys, who were aged 11 or 12, in her residence, the sick bay and a tent while the boys were camping on school grounds.

She pressured one boy by telling him, ''Come on, let's go. Don't be a puss'' and told another boy she loved him.

Some of the assaults occurred with other boys either watching or keeping a look-out.

The woman's treating psychiatrist, Mike Richardson, told the court she suffered from bipolar disorder and ''was unable to control her libidinous impulses whilst in a euphoric state''.

A second psychiatrist, Olav Nielssen, diagnosed her with a bipolar mood disorder and said in almost 20 years he had never come across a similar case.

''This was just so out there, so uninhibited,'' Dr Nielssen said. ''There's no other explanation than her mental disorder.''

But a psychiatrist called by the Crown, David Greenberg, gave evidence that the accused had paedophilic tendencies and knew the wrongfulness of her behaviour. She rationalised it as ''providing sex education''.

Judge Garling said that while it was unusual for a female to sexually abuse boys, her ''bizarre behaviour'' did not mean she was necessarily mentally ill.

Over the five months last year when the offences were committed, the woman, who was married at the time, lived an otherwise normal life without exhibiting behaviour typical of a person in a manic state, such as going on spending sprees, he said.

That the woman would lock doors during the incidents, spoke of going to jail if discovered and told one boy they had to go somewhere secret and safe, was evidence she knew her actions were wrong, he said.

''It's quite clear to me the accused was a lady with limited intellect who became attracted to one of the boys in her care, who she believed she was in love with, and that she should not only carry out sex with him but with his friends.''

The woman was stood down by the headmaster after a victim told a staff member.

The woman, who was supported in court by family members, rocked slightly back and forward in the dock as the judgment was read out yesterday. She remains on bail and will be sentenced in March.