I agree with position28. Your next conversation with the teacher could sound like this:
"You and I talked about Billy on January 28. At that time, I was concerned about his behavior with my daughters, and you committed to taking care of the problem. It is now February 25, and the problem still exists. What can you tell me about this? What actions have you taken? Why haven't they worked? What is the necessary next step to take care of the problem?"
See what you learn by asking questions. Perhaps the teacher did take action, and Billy just got more secretive (and maybe more angry in the process). If you are satisfied that the teacher did what she could, and will try to help again, then don't escalate. But, if you think she didn't act appropriately, tell her that you are concerned and you would like to talk to the principal.
You'll need to follow the same approach with the principal, but make it clear what your intentions are if the problem is not resolved (transfer your girls to another school, take it to the superintendent, take it to the school board, or whatever). While schools these days do have policies on violence, they also have them on problem resolution, and so there will be steps they have to follow to take care of bad boy Billy.