If you feel like going stealth is something that's possible for you (and there won't be any catastrophic events if you are outed, i.e. getting beaten up or the like), than you could probably pull it off pretty well.
On teachers/subs and nicknames; normally teachers will mark on their roster if someone has a nickname they like to be called, and so if a sub is there they'll have that on there. Not guaranteed, but that's generally how it goes. Alternatively, you could register for your new school with your male name, and explain the situation so there are no mix ups.
Hopefully it'll work out for you. 🙂