How to prevent being injured/abducted by ex?

I am in highschool and my soon to be ex boyfriend recently turned 20 and im still under 18. He's emotionally abusive and manipulative, and will do anything to keep me from leaving him. He moved away about five months ago(out of state) but is coming back in about three weeks. He says he is going to come visit me at school but I really want nothing to do with him. Im afraid that by breaking up with him that will push him over the edge and he'll try to hurt me or abduct me because of his" if I can't have you no one can mentality" how should I approach this and how can I protect myself? Thank you so much I really appreciate it.