You need to tell your Parents, the School and the police.

His behavior is not okay. EVER!

Write down as much as you can about what you remember about where, when, and what happened and who else was around. Block him from contacting you on social media. Tell your parents today. If you don't feel comfortable telling your parents, tell a teacher, school counselor, parent of a friend, religious leader (if you have one), or some other adult. You need to keep telling and asking for help until something is done. It doesn't matter that he was your friend. If he does something like this again yell really loud "stop, take your hands off of me!" try to get the attention of anyone else who is around. It may feel weird, but it brings attention to what is going on.

I know it can be hard to talk about these things, but it is important for your safety that you speak up.