THE MOTHER of an 11-year-old boy who was sexually abused by a house mistress says she was told by her son's exclusive school to keep the matter quiet.

The school, which cannot be named for legal reasons, also failed to organise counselling for the young victims or offer their families meaningful support, the woman told the Herald outside the Sydney District Court yesterday.

Cassandra Leigh Johnson, 42, was sentenced to at least five years' jail for having oral sex and sexual intercourse with six boys, who were aged 11 or 12, over a five-month period in 2009.

Judge Anthony Garling said Johnson, a petite, youthful-looking mother of two who lived on the school campus at weekends as a ''house mother'', took advantage of immature and naive boys who were ''in no position to deal with this predatory behaviour''.

He said in sentencing her that he took into account the evidence of two expert psychiatric witnesses who said she had bipolar disorder, but he did not factor in her gender.