I think Cantexplain is right; you should take this step by step for your own comfort. My only input (as a medical provider) would be that you should start with talking about the pills. I think this would be a good start for you, since it isn't really about you directly- think of it as just some medication someone prescribed you- and also b/c I think it would help her.

I second what everyone else has said. A school therapist has no power to kick you out of school or your home. All she's gonna want is to do what's best for you. And you have A LOT of say in that!