How can I see a therapist without my parents finding out?

[Grace]

As a teacher you are a mandatory reporter, so if she thinks you are going to seriously hurt yourself it is her job to report it to important people. This teacher obviously cares about you, I think you need to talk to her about your parents and the trouble that you're going through.
I was exactly the same at your age, I had chronic anxiety, depression and was bullied constantly. I wish i had reached out to a teacher who gave a $h**, then I may have been able to get better a lot quicker. Continue seeing the therapist, and be honest with the therapist and your teacher. Your parents obviously don't take the whole thing seriously so you need to surround yourself with people who do. If you're honest to people like your teacher about how your parents are around this whole scenario then she may stop telling them things, unless you're in danger of seriously hurting yourself. If you really want to get better, then a good therapist will help you with the issues, because its your way of thinking that causes these issues. Life can be completely different.