She pretty much only ever wears clothes that covers her arms and legs. She wore a t-shirt for the first time in ages and I noticed she had red cuts on her wrists. I don't want to ask her about it. Everybody is really horrible to her at school but to be honest, she's never been very nice, picking on people simply because she dislikes them. However others have been picking on her, calling her a cow and telling her to get friends (she only has me and another boy). I don't know if her parents know or if she's doing anything about it but I was wondering if there was anything I could do to help her. Thanks.
there is but she may not thank you for it. Ask her to talk to someone, preferably a doctor as a start, no idea if you are in UK, but there are places where she can get help. I have come across any number of young people including boys who self harmed, they like your friend need professional help.
If you have a good open friendship then simply tell her you saw the marks and ask if she wants to talk.
Don't get flustered or panic just yet, it is very common , see how she reacts and encourage her to seek some help from Parents or Pastoral care at school/college.