I agree that you should let the school know what's up. You're daughter shouldn't be punished for something you do that isn't even wrong to begin with. I honestly think I should avoid these sort of posts, because I always get really jealous. I still have to wait just a little bit longer before I can start HRT. I have, however, worked my glutes to the point where I can crush walnuts with my butt. I'm strong, but not huge, so my butt really stands out. Little do people know, "lady butt" is among the most flattering insults I've ever heard.
You will have to come out to everyone at some point, though, so you might as well deal with it now. You could try to hide everything, but constantly having to switch presentation just sounds tiring to me.