Well, considering it's high school, I am willing to bet you won't find many straight, cis males going through the trouble and getting the reputation as someone who is transitioning to female just to be able to see girls walking in and out of closed stalls inside the bathrooms. And I would imagine if it's just a dude throwing on a dress in a mockery of the rule, they would be reprimanded the way a female student who admits a false rape accusation is.
I mean, just from a logical standpoint- most sexual predators don't want to get caught and unless they do an amazing job (which take a long time) of looking convincingly enough like a cis-gendered woman, they are going to draw attention to themselves just from pretending to be trans. Plus, this all implies that we haven't really cared much about all the young boys being victims of predators in bathrooms.
I totally understand that fear for your child makes you want to err on the side of caution, especially if trans is something you don't believe in or agree with. But, statistics don't lie- most trans individuals have been using the bathroom with their gender (not their birth sex) and you are hard-pressed to find instances where a trans person went into a bathroom and sexually assaulted a child. You do, however, find staggering statistics about cis-gendered men who dress as men sexually assaulting children.
So, logically (and statistically) the fear shouldn't be trans individuals' right to feel comfortable with something as basic as going to the bathroom, or even cis-gendered men pretending to be trans to get into women's restrooms, but it should be vigilance towards cis-gendered men dressed as men using bathrooms.
That all being said, I know where I'm posting. I know this will fall on deaf ears, and will honestly just fuel the near-constant ridicule of liberals, hippies, freaks, or pick you choice of label. So whatever, I just genuinely hope the people who are spewing the most hate never have a trans child. They will have enough to deal with in their life journey.