A former house mistress at a prestigious school who was on Monday sentenced for having sex with five male students had slept with more boys in her care.

NSW District Court acting judge Christopher Armitage acknowledged the "extraordinarily lenient" two-year suspended jail term he handed down to the 25-year-old woman, who had regular sexual liaisons with boarders at The Armidale School in late 2014 and 2015.

"I hope I have not made a mistake in the sentence I have arrived at," Judge Armitage told the packed courtroom in the Downing Centre, adding it had been the subject of "considerable reflection and sleepless nights".

The staff member (centre) from The Armidale School outside court. Credit:Brook Mitchell

The court heard a "toxic culture" had developed in the boarding house, in which the young woman slept with students aged between 15 and 17 after inviting them into her room.