I have a friend who I think is depressed, but she seems happy all the time. What do I do?

I have a friend who is also a teenager. She seems like the always happy and cheerful type, but she doesn't have a great home life. Her parents expect more from her than she can live up to, and they punish her harshly (no physical abuse involved) when she disappoints them. Last February she mentioned suicide, but the next day said she was over it. The other day I noticed cuts on her wrist, right after we recieved report cards. I'm very worried about her. What can I do?