Hey, Troy. We're pretty much in the same boat here. I'm going into 11th grade too.
A lot of people in my school know I'm trans, but none of my teachers know, as far as I'm aware.
I waited until I tried committing suicide to come out, so...I don't reccommend waiting if you know you can't wait.
I don't have gym anymore, but if I were you, I would email the principal or guidance counselor about it and see what they say. Why ask to change in the bathroom? Do it anyway. I always changed in the bathroom stalls in the girls' locker room.
You might want to know that coming out doesn't always go well. You most likely will get some harsh comments like I did when first coming out (I didn't come out to my teachers, though. Everybody just kind of found out, really. Once I started telling people I'm a guy, they had this sort of revelation, like, "WAIT, BUT I THOUGHT YOU WERE A GIRL." People get over it eventually though because most don't understand it.
Good luck. (: If you need someone to talk to, feel free to message me.