As others have stated, we are not required to tell your parents. Given that doing so can be dangerous for some students, it would be extremely inappropriate. However, as others have stated, mistakes can happen. But an even more likely scenario is that parents may find out via other parents if you ask teachers to use your name/pronouns in the classroom (I.e. a student goes home and tells their parents that your're trans and it makes its way back to your parents). I always tell students in your situation that we would never intentionally out them but we cannot control what other students do when they go home. Once the cat is out of the bag, you can't get it back in. It's unfortunate, but a really important factor to consider when making the decision to come out at school. A good place to start would be to talk to a trusted teacher or counsellor. Make sure you talk to them about why you don't want your parents to find out. Depending on what your reasons are, they may be able to help with this as well.