Look, I had two brothers. I was their sister. Even before puberty but especially afterward, they didn’t want me walking in on them, and I didn’t want them walking in on me.
Furthermore, that was the case in a home restroom.
In a public men’s restroom, there is a wall of urinals where men must partially disrobe and reveal their genitals in order to urinate. I’m sure that things are already fraught enough in a school full of fragile young egos jostling for their place in the world.
But if a teenage girl (even if dressed as a boy, and even if showing some testosterone-caused features) walks into a boys’ bathroom at school every day, no teenage boy in the world is going to be able to just ignore it and go about his eliminatory business. Even if they tried, they would either be unable to pee because of sudden urination shyness, or they would find themselves aroused and embarrassed. Boys with a bullying personality will react by trying to beat this girl up or sexually assaulting her, of course, while normal kindly boys will probably react by abandoning that particular bathroom’s use to the girl… and the bullies, though the decent boys won’t realize that. If the school realizes this, they will have adult teachers chaperoning the bathroom and preventing the bullying; if they don’t, the consequences will be very bad.
So yeah, at best this girl will have her own boys’ bathroom all to herself, while the boys double up and beat themselves up mentally (and probably have an increase in fighting among themselves, in response to crowding and self doubt). There will probably be other bad consequences, because school is an unpleasant environment at best, and bad things tend to happen there.
Furthermore, any time you say that manhood is just a matter of taking sufficient testosterone, and particularly if you back that up by adult authority, you also undermine the social and sexual development of every boy in that school. Teenage boys are really fragile when it comes to matters of manliness, which is why girls have to be tactful around them (without being seen to be tactful, which is annoying but necessary).