I might need some psychological help after this...

My ex-bf was abusive towards his friends. I've loved him for three years and we had gotten together then split apart and had recently gotten back together. Almost two months in, he asked to be in a polyamorous (more than one person) relationship with two of my friends and I. One flat out refused, but the other was the one who SUGGESTED THE IDEA IN THE FIRST PLACE. He said he "Didn't think it was right to just choose one." And me, being my doormat self, said okay. My bff slapped him when she saw him next. Keep in mind, I was a seventh grader. So days go by, I stop eating entirely out of stress, I can't sleep, and I'm keeping my whole relationship a secret from my parents. One of my other friends talks about committing suicide from the stress of it, so her mom tells my mom. It was not easy explaining this to my mom and dad. So my mother contacts the principal, and he's not allowed to talk to me anymore. Just before this happened though, he had literally threatened to kill me and had been cruel to me before with violent mood swings. He still tries to talk to me. A few days later, I find out that he and his girlfriend had SEX. THEY WERE BOTH TWELVE! And though she may identify as a boy, she's still a girl. Then, during summer break, he tries to anonymously contact me on my social media and question me. His birthday was in his username. I saw right through it. And this year, I have three classes with him unlike last year's one. I know he's going to, but I don't know how to react when he tries to contact me. Help!