I don't know whether or not you should tell the school about it. I think that would depend on what their potential reaction would be and how she would feel about it. It could have some serious consequences.

If it were me, I would just make an effort to befriend her and not bring up the cutting thing in the beginning- just talk to her about her life, ya know? And eventually the cutting thing might come out. Who knows, maybe she would tell on her own?