Jenny -
So sorry to hear about this!

Hope your sweet DD is OK!
This is no longer "bullying", this is "Assault and battery". I know our district classifies bullying as things like name calling, intimidation, behaviors designed to isolate a particular staudent, etc. Anything that involves an ATTACK is much more serious! Even for THREATS a student here would have to go through a "risk assessment" to determine what sort of action needs to be taken to protect the other kids.
Call an attorney to find out what sort of things you can do. Those girls need to be sent to an alternative school, sentenced to community service, etc.