This is by no means legal advice, I am here seeking my own. However, I'm just going to throw in my "parental" advice... from one parent to another. First and foremost, you didnt state the ages of the kids involved.
What *I* would do. Even at this point, if you didnt report this initially. What *I* would do is inform the school of the incidents. The school wont discipline because none of this happened on school grounds, even if it involves school children. However, the school should be made aware because they will watch more closely the "goings on" of your daughter and these other perpetrators at school. That is more for your daughter's safety. You should also report this to the police, if for no other reason than the police should at least talk to these other girls, which in a perfect world, would scare the crap out of them. I doubt you'll get any kind of charges and/or compensation, but I would raise holy hell with every person possible. The police should make the parents aware of what their kids did, and they may get punishment there. Starting Monday morning, I would be on the phone with 1) the police and 2) the school.