This is going to sound really weird however...

I would rather my daughter be on the receiving end than on the giving end as the child that is acting like this has real bad problems I would not want to be dealing with.

As it stands you have no choice you need to make a list of all the things that have gone on and make an appt with the head teacher handing the list them them.

You cannot afford to feel sorry for this other child or her family. You have your own family to worry about and this could get serious with one of your daughters getting physically hurt.