Your poor DD. I am so sorry she is dealing with this. I think you have done good things so far and agree with a lot of the advice you have gotten from PPs. I would add that you really should document who you talk to at the school and what protections they say they will put in place. Follow up in person meetings and conversations with emails, even if they are your friends. Document the heck out of this. If it escalates and you do end up needing to involve the police, you want evidence of what has been going on and how you have tried to address it.
Also, your daughter should not be engaging this guy physically. I know she is taking up for herself, but I really agree with others that it can backfire. Not to mention, this is school. She should not have to use physical force to protect herself! Please make sure you get a face-to-face with the top administrator ASAP. And follow up with everything they promise in writing! I hope this is resolved quickly. No kid should feel this threatened at school.
DS - Feb 2010
DD - May 2012