Tell her to say tehillim and remember who she is and what she believes in. Remind her that she is normal and that her hormones are working. Maybe a little overtime but she should not allowed her fantasies to run out of control and make her do something that will change her life and make it spin out of control. She should understand that the choices she makes today will define her future and the paths those choices will lead to.
Encourage her to speak to the guidance counselor in the school or her own private therapist so that what she tells them is confidential and won’t get back to the teacher or the principal. She needs a professional to talk to that can guide her. She is NOT going to listen to her friends who she will think are immature and not on her level and she for sure won’t discuss this with her parents or mechanchim who she feel for sure won’t understand.