My response to your question, regarding whether or not it is your fault that an adult man is raping you, is emphatically no. It is not your fault, no matter how you feel about the experience. Please do not blame yourself. No one ever deserves the type of treatment that you are receiving. No one. Ever. Never forget that.
The man who is forcing you to have sex with him is a predator. His crimes need to be reported to the police as soon as possible. What he is doing is illegal in all 50 states across the country. He, in all likelihood, will be sent to jail and that is where he belongs. Once in jail he can no longer harm you or threaten to harm you.
I would strongly advise you to call the police, tell your parents, or tell a faculty member at the school. Here is the phone number to the Child Abuse Hotline in your city: 800-252-5400.
You cannot keep this a secret. Do not consider his feelings. Go immediately to an adult or to the authorities and report what is happening to you. It is the only proper way to handle this situation.
I’m sorry this is happening to you.