My ex boyfriend is verbally abusing me at school.

What do u do about my ex boyfriend basically screaming at me and finding ways to do so in school and bringing all of his friends along to intimidate me. He knows my weaknesses and he seems to be using them against me, how do I react towards thi? How do I make him stop? It’s seriously upsetting because I’d given him no reason to leave me in the first place never mind have a massive go at me. HELP