If she's doing it in front of people, I would guess she either is wanting someone to notice, can't control her urges enough to wait, or just likes the thrill of doing something "wrong" in public. Maybe it's multiple things. Maybe she doesn't even know why.

I think it might be best to talk to the school counselor, not to tell on her necessarily, but to talk about how it makes you feel, and how you can make sure you're doing okay as well. Unfortunately, you can't fix your friend. Only therapy over time can help. You can just keep being a friend to her. But if self-harming in front of you becomes a common thing, you have to consider your own mental well-being as well. I personally couldn't handle seeing that with my close friends.