I want him to stop ! what do I do ? (Long)?

My ex is scaring me so much and not sure what to do. He touches me anywhere he can when he gets the chance. He is in every single lesson with me even p.e and he thinks he can get away with it all. In p.e he tries to sneak into the changing rooms and today after work skills I stayed behind to sort my books out and he walked up to me and tried to kiss me but only manage to kiss my hair... he then went right up close to me so we were touching and he was holding me and his mates were behind him laughing and they told me they dared him to do it but he does this alot. Sometimes he grabs my bum and my boobs when he gets the chance. I really can't take much more of this. I suffer from depression,anxiety and paranoia and I had eating disorder in the past so this is really not helping me. I told him no so many times but he doesn't seem to hear me and all of you may think this is stupid but when he does these things I go so weak and I start shaking so I don't find the strength to push him off. Also today my teacher witnessed something but all he did was say "don't speak like that" this was after he touched me up and he said how he wanting to s***g me but teacher overheard him. I don't get why he seems to do these things to me without teacher even knowing?! In P.e he kept grabbing my bum and no one seem to noticed? I want to tell my mum but I know for a fact she will laugh in my face and yell at me for being a wimp. If I tell a teacher they will end up ringing my parents anyway to let them know. I am alone here :/ I just need some advice on how to stop him before something else happens. I am 15 he is 14.. he is quite big and like I said I had eating disorder in the past so I am not the strongest.