Talk to other parents. There’s a non-zero chance that your child is not the only one. Also, for children that young, a lot of the times such behaviour comes from negative home experiences - be it their own parents bullying them, or even just neglect (no corrections, guidance, etc), so you can’t always expect help from the kid’s parents. However, if you manage to get the involvement of other parent(s) then it becomes more serious, and hopefully will have the staff listen and act.