Take it as far as you can! Hugs to your family, these brats did this because they knew their parents wouldnt do anything and they probley wont again now, but the school must and the police should. They would be expelled if that happened around here. (I think alot of us can say we would rather be in jail for something than face our parents if we got in trouble growing up, unfortunatley that was never instilled in these brats!)
Edited to add: Make sure the parents of the brats split your costs on the emergency room and such also! They might not care much but it might get their attention when they are paying your DD's bill for it!