Call the police and report it. Or if you have a counselor or police liason office at your school, go see them=they are mandatory reporters and will help you report it.
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Have you informed your parents? They could be of great assistance to you in a very difficult and traumatic period of your young life. Explore whether there are in rape crisis centers accessible to you. You are awfully young to be shouldering this burden on top of being one of his victims. Get assistance ASAP. Good luck.
First, if you have a school police liason officer, contact him or her. The school nurse would be my second choice for a contact person. Both of them are in a position to help you with the next steps.
Second, given your age, the likely charge is 2nd Degree Sexual Assault of a Child. If the prosecuting attorney feels that "force" was used or threatened, then a relatively new "charge enhancer" would make it a 1st Degree Sexual Assault. Assuming the 18 year old were ultimately convicted:
If the use of force resulted in sexual intercourse, there would be a mandatory minimum 25 years IN PRISON plus supervision. If the use of force resulted in sexual contact, there would be a mandatory minimum 5 years IN PRISON followed by supervision.
First things first: Report the crime, Get your own professional counseling to deal with what happened.