I worked for the coach at my academy (private high school) and I had to clean the boys and girls lockers. The girls had privacy. The boys had communal showers. With all of the concern in the Christian world about homosexuality, that never made any freaking sense to me. This was a boarding academy and the dorm showers were the same. Again, no freaking sense.
So now, consider taking your "logic" to its natural conclusion. Should a girl who thinks she's a boy use the boys locker room? What do you think is going to happen to a girl who thinks she's a boy if she's in a communal shower with a bunch of boys? I mean if we're going to break down all of these "gender barriers" and have people uses the facilities that best fits who they supposedly are then she (he) should be required to use the boys facilities right?
Here's a crazy idea. Delay sex reassignment treatment until you finish high school. It's not going to kill you. Obviously even some old fart like Bruce Jenner can be "reassigned." Seriously, there is enough social pressure in high school that there isn't a good reason to add to it by changing genders. Or move to another school where nobody, including the teachers, is going to know you are really a he or a she. Seriously this can be pulled off. The federal government now bans schools from requiring birth certificates for registration because that's somehow "unfair" to undocumented immigrants. (See:
http://www.dailyherald.com/article/20140520/news/140529963/) So....move somewhere else, register him as a her, make sure that he does everything to make sure nobody ever finds out, problem solved.