Does the school not have a psychologist on staff or a counselling service on retainer. Certainly there must exist a protocol and standard operating proceduref for just such situations.

the best thing you can do is put the student in touch with resources - starting with a doctor. As she is above the age of 16 she can see and receive medical treatment without her parent's consent.

As her teacher and responsible adult you are bound to take action. This would entail contacting authorities. Doing so is not a career limiting action.

It sounds like you want an intervention. This MUST be done with two additional people present. One, your union rep (or else a third party who can verify your side of things). Secondly, you need to have an advocate for her her as well. Do not attempt to do this on your own as it could backfire and you could face a great deal of legal trouble.