It's not that I think that's inappropriate, but you have to remember that the school has the upper hand, when you look at that solely as a student vs the administration. Unfortunately, a tee shirt is just a political placard in motion on the canvas, and whether it's true or false, offensive or an advertisement, as a press artifact it meets the definition of propaganda by virtue of being a single poster sized image that can be easily reproduced by mechanical means quickly, and that's that.
I've seen ruder tee shirts than a simple slogan like "only two genders", and I've seen both worse political messages, and age-inappropriate shirts on people. (Playboy, beer, cigarettes... maybe, except for Playboy, but not under 21 in school.) Another big hot button is the band tee shirts, it's advertising propaganda, but you don't know how many records they've made or what they sound like. The school always wins that argument. The problem isn't that I can't wear a Coca Cola sweater, it's the fact that some political propaganda si allowed, some is enforced, and some is sent home. Any it's always on the administration, the court always finds for the school. Parents and kids can never strike back at competing propaganda on other kids, because the court doesn't care, it's the school's rules and there are a million schools.
I'm not saying the school is 100% correct, I'm just quoting the law.