I hope that you can talk with one of your parents (though I know that's not always possible) so that they can help make sure he doesn't hurt you. If you can't talk with them, maybe try talking to a guidance counselor or a friend's parents. It would be best to get a trusted adult's help on this one.
If he threatens you at all, don't hesitate to call the police. Then it will be on record and if he tries to hurt you it might be easier to prosecute him.
Don't let fear control your decision or make you afraid to leave him. That's what he could be counting on, and he might try to intimidate you into staying.
Take precautions, especially at first until he cools down from the break up. If possible stay away from places where you feel that you'll likely run into him. Don't go out alone in case you do run into him. If you do see him, ignore him and get away fast so that he doesn't have the chance to hurt you.
The important thing is: Get rid of him. Good Luck!
See Crisis Center / Rape and Crisis Center / Peer Violence
-Erin-