possible sexual harassment idk? Help please?

I am thirteen. I have a boyfriend of 10 months and we are happy and healthy. A few months ago (maybe about 5) I was in math class and the school had just handed out our school newspaper. Well all the boys in the class being actually stupid rolled the newspapers up and started whacking each other. My boyfriend participated but I didn’t care because boys will be boys. Me and the girls laughed about it until this happened. Some dude in class ran up behind just me and whacked my butt. Not on accident. I turned around immediately and glared at him but didn’t say anything. I’ve told my friends and boyfriend about it but idk how to tell my parents or if I even should. It hasn’t affected me too bad. Just makes me upset at myself for not calling this dumb kid out immediately. This kid is also a well know CREEP at school. Idk guys I just need some guidance.