Does she have access to a counselor or a emphatic teacher to lean on or talk to? Maybe try and get her to open up to an adult about some stuff will help the both of you not have to handle so much alone.

Can you tell her parents about her cutting? do they know something's up with her? maybe they can help her?

P.S. i know i haven't been on here in a while etc, so feel free to take my opinions with a grain of salt (if that is the proper expression)