I don't know how far you are into school or what your school is like but I find schools to be a little brutal when it comes to orientation or gender. As you've said, you've been bullied a lot in the past and I think you coming out as trans at school will only enhance this. This isn't to put you off coming out as I understand this will make you feel far more comfortable, it's just how I view it. Overall it's your final decision and if you feel coming out at school regardless of potential bullying then go for it.
If you do decide you still want to come out at school you could try various ways, over Facebook like you said or you could even do it virally through asking your friends to help you out by spreading the word to their friends, and their friends spreading the word further though it'd be a matter of trust in this case.
Your idea does sound best though.