making someone sign something with any type of threat/intimidation/coersion is duress and it negates what was signed. Tell her to sign it (so she will not gain more problems) but sign it as: Jane Doe signature acknowledging reciept only.
OR:
Jane Doe under duress

If they buck explain to them many stats have laws prohibiting someone questioning a person under 16 w/o a parent you support your daughter and any further communication regarding the document should be made directly to your attorney.
your daughter did the right thing trying to seperate them now what is the school trying to teach her about doing the right thing.