How do i tell my mom that i got in trouble at school?

I'm glad you decided to tell your parents, and not do something that will get you into more trouble. Maybe say something like, "I need to talk to you about something that happened. I made a mistake and I want you to hear about it from me." Regardless of how it starts, it's not going to be a fun conversation. It will, however, make you look mature and possibly reduce the amount of trouble you'll get in to tell them yourself.