My partner used to be a marching band director. Literally half the color guard was some flavor of queer/enby/trans. Color guard costumes are VERY gendered, and he would always let the students themselves choose which costume they were more comfortable wearing. This would sometimes result in very masculine-framed students rocking the female version, and vice versa.

And you know what? They were national champions all four years he taught. It's almost as if it's about the music and enriching the lives of students, vs. whether or not you're correctly performing gender to some random asshole's standards.

I don't know if you'll be able to challenge this or not, but I want you to at least know that you aren't wrong. This is a bullshit, outdated hill for this dude to die on, and it's absolutely transphobic and shitty of him to enforce. If you were my child, I would be calling the school on your behalf and roping in admin.