Sexually assaulted in maths (regularly)

(I’m 14)So I sit in front of this guy who is my age but he does weed and drugs and sells weed and has sex and this that and the other. He always says “oi (my name for this will be s) S let me star fish you” or “S let me finger the fuck out of you” or “s let me take your virginity with that wet pussy of yours” i hate it. He’s not meant to sit behind me and sometimes when he tries to hide behind the teacher he will duck down behind my chair and touch my ass and stare straight at it and say “let me destroy that” and shit. It’s honestlyy the worse thing ever and since he does drugs and stuff I don’t wanna get on his bad side. Iv asked to move countless times but my teacher won’t let me. Help