13 yo girl, feeling helpless

In art last year, a boy in my class started lifting my skirt up. I told him to stop. He ignored me and continued. I asked him again and he still carried on doing it. After me telling him “stop” twice more he briefly touched my bum. His friends were laughing. I (stupidly) left the room crying as I felt so powerless and was unable to stop it. Now he always makes jokes about it, calling it “sexual attractions” and saying he “tripped”. I’m now really stressed out and paranoid about it happening again, never mind him joking about it constantly. I feel like an idiot for crying.