FWIW I have had good results in the past by putting my concerns in writing to the principal with class teacher(s) and school counsellor cc'd in. I described the entire incident as reported, explained the ways in which it was unacceptable, and the desired outcomes. The principal later told me they were very grateful for the letter because they were able to give a copy to the parents of the 2 children (with my child's details stripped out). One set of parents had been in denial that their child's behaviour was unacceptable, but the calm and detailed letter gave the school a tool to work with them.