Middle school to high school relationships

So I'm now a freshman at my high school and during my last year of middle school, I secretly dated a guy because my dad doesn't want me dating until I'm about 16. (I'm 14 right now almost 15) This guy that I dated had a girlfriend before me, broke up with her to date me, we dated for 3 months and during that, he was depressed and that started to rub off onto me and make me depressed and suicidal. He left school to go to a place that helps him with that and also help him learn but he didn't like it and wanted to be back at school with me. But while he was gone, I was much happier and early to never depressed or anything. He comes back so lost feelings for him and he cried bc he acted like a baby all the time and I was finally sick of it. He then goes back to the girl he had before me and hasn't talked to me bc he was mad that I'm not dating him anymore. He wants me dead. I had a freshman focus and was with a friend of mine and he was sadly in my group and neither me or my friend are friend with him now, so he will try to make amends with my friend but would insult me infringement of him and would make a sign like he wanted me dead. It was ridiculous. Now it's High school and I sadly have him in 3 of my classes and I thought I overheard him say stuff about mine and his past relationship and possibly saying things like I was a bad girlfriend and saying stuff that showed I was a bad girlfriend but im not fully sure about that now. He would constantly stare at me and class I feel so uncomfortable around him and in class unless I have friends in my classes. I now have a new boyfriend that I am secretly dating since I'm not allowed to yet but I'm not telling my dad until I'm 16 that I'm dating. Does anyone have any advice on what I should do about my past boyfriend on how to handle having him in my classes and how to avoid him or something? Any kind of advice will be good for me. (Sorry bout that really long past relationship story.)