SH (self harm) can often be seen as similar to binge/purge and drug abuse, other addictive behaviors.
For many people, it is something that will be an issue (at least off and on) their entire life. I've never been a drug addict, but I've heard lots of stories where during a stressful time, or when confronted with a trigger
Point being-even if she stated that she resolved to quit, you are right. Relapse rates are very high.
I'm not sure it's likened to OCD, unless she habitually cut herself only before/after an exam or something like that. I think it's more like a self medication, as the PP mentioned. Pain really does release endorphins (think of childbirth!) and really does physiologically numb emotional pain.
I have heard of/seen Zoloft and Paxil (though not together) being used to treat SH, as well as an anti-anxiety med (Xanax, ativan, etc) for acute episodes.
NOW-Before talking to the girl's parents, what about encouraging the girl to talk with her mother in the teacher's presence? Or refering her to the school guidance counselor who can decide if this is still an issue.
As far as teh legalities....I'm not sure.
Hope this young woman gets some help, either way.