Self-defense classes. I mean it. If she knows how to defend herself and how to neutralize an attacker long enough to get away, she won’t be afraid.
And just explain that all of you were obligated to go to the school when it seemed there were drugs involved. That could not be changed. And say, “When people get embarrassed, sometimes it makes them angry and they lash out. This girl would have done this to someone eventually, regardless of anything you did.”
Tell the school about the threats of violence, and inform them that if your daughter is physically attacked, the time for dealing with it in school has passed, the police will be called and battery charges filed.
Sorry about all this!