Dean Groves, whose two daughters aged nine and eight attend Herbert Thompson junior school in Cardiff, is backed by Welsh-based anti-bullying pressure group, CURB (Children Under Risk from Bullying).
Earlier this month Mr Groves' older daughter Jessica was threatened by a boy brandishing a knife from the school canteen.
Mr Groves said he wanted to find out what the legal position was when children were put at risk by other children on school premises.
"I know the head and the staff are very restricted in what they can do, and the governors perhaps les so - but to suspend this boy for just five days, as they did, is no more than giving him a holiday," he said.
Maureen Booth-Martin, from Cardiff, who founded CURB, said: "We are very concerned about the duty of care owed to children by schools which are 'in loco parentis'.
"If children were threatened or bullied like this at home there would be child protection procedures. We cannot identify similar safeguards in schools."
Cardiff Council confirmed the incident but a spokesman said: "No adult witnessed the event. The boy was suspended and has not returned.
"As far as I know the father of the girl has been to the school and seems satisfied with the current situation."