Don't tell the police at this stage, if it turns out that he isn't going to punch you then you could get in trouble. Do this instead:
1. Explain the situation to a parent, but make sure that they don't tell the school yet.
2. Stay near people you know and trust, so you have witnesses to anything that may happen. Obviously try and avoid him, and don't do anything to provoke him. If he confronts you verbally, try and reason with him.
3. If he punches you, CALMLY do the bare minimum you need to do to stop him hurting you again before an adult arrives (You can justify this as Self-Defense, don't throw him on the floor, kick the crap out of him and start swearing your head off at him, just defend yourself until he stops coming back at you).
4. Obviously you will be questioned by your Head Teacher about the incident, so just calmly explain exactly what has happend, and inform them that you have witnesses. Get your parents to talk to the school as well, seeing as you told them what was going on BEFORE he decided to punch you, it helps put you in the clear.
Have you done anything to provoke him?