Protect Cis Girls
If this law is sustained, then at best any boy who is actually under the idea that he's a girl can enter the girls room, which is bad enough, causing them natural inhibitions about carrying on their personal needs. And at worst, any male student who claims to be a girl for less than chivalrous purposes, can enter the girls room. It'd be one thing if there were at least some tests to determine if the boy is one of the former, as he won't be as troubling in actuality. But if it's a No Questions Asked policy, then the sympathy of the school should lie with the children whose' sex and gender are, in every sense of the word, female.