This boy likes me!!!!!!!!!!!!!!?

Okay, so I'm in the 8th grade I'm 13 years old and I'm a girl. And this boy at my school in the same grade as me likes me. And the boy is mentally retarded. It's not like extreme mental retardation it's very mild, he can talk and walk but he talks kinda funny and he doesn't act "normal" well.... he looks normal on the outside but if you start talking to him you can tell that he's retarded, but that's not the point. He likes me and he's REALLY starting to creep me out. He follows me and my friends home from school. And he's in the same homeroom as me, and we got our seating charts changed today and guess what??? He sits RIGHT behind me!! So in cl he touches my neck and stuff. I've told the teacher many times and she just tells him to stop and that's it. And at lunch time he tried to sit next to me but i told nicely him he couldn't because my friend was sitting there and then he started throwing a fit and he slammed his lunch on the floor and it was really embaring. And at recess he "touches me!" It's really scaring me. I've told him MANY times to get away from me. And I've told him many times that I don't like him. What should I do?