Student who won't eat?

If a student (Age 15) wasn't eating at all; starving herself and the very rare occasions she does eat she vomits. what would a teacher do? What if the student always felt sick and anxious and light headed, she slept in class a lot and her once A grades became B and C grades. She's always so sad, like severely depressed, and is extremely fragile and quiet. What would happen? And what if she were to collapse at school. How would she be dealt with? She alsO self harms, badly. Gashes all up her left arm and leg. Although it's usually covered. There have been many class partys, etc with food and she won't eat anything she doesn't even look at the food. I go and offer her some and try make her eat but she refuses. She's gotten really pale too and frail. Please and thanks for your help.