My friend really likes this guy in my class, but she's pretty sure that he likes me. I don't like him at all and recently she started cutting herself. What can I do?

Nothing. She has mental issues, and she needs help, more help than you can give her. She needs to tell her parents, and she needs psychological help. Don't get involved, you'll only end up being disappointed, because there's really nothing you can say or do to stop her from harming herself.

Talk to a trusted teacher at school. Do you have one?

You can tell someone in authority what she is doing. Do you know her parents? Speak with a counselor at school and ask for advice. Is she doing this over a boy or does she have other problems?

You have to tell a trusted adult either your parents, a teacher, school nurse, or counselor so she can get help for her problem.