So a new high school and stadium is being built in my city. Is there any documentation on making new buildings trans friendly that I could send to the school?
I don't have anything off the top of my head, but have you tried googling something along the lines of "gender neutral architecture"?
The only issues I've ever seen coming up relate to bathrooms and changing rooms. It would be absolutely awesome - and newsworthy - if you could lobby the school board to ensure that there were a few gender-neutral bathrooms (call them family bathrooms if that helps get it past the weird politics of school boards these days), and at least one gender non-specific locker room for students. None of this stuff would necessarily cost much money, but it would make a huge difference in the lives of trans children who are at an extremely delicate stage of their lives.
It would also make the school boards and parents reveal their true motives when they try to stop trans children from using the correct facilities in schools. They wouldn't be able to claim that it's to protect the privacy of the other kids anymore. The only reason for stopping something like this would be pure bigotry, as the usual "we don't want Peeping Toms" argument would no longer apply.
High schools are often built to rather formulaic plans to save money - a county or district might use one set of plans for all their high schools, allowing for minor tweaks here and there. But I honestly see no reason whatsoever why reallocation of small areas in the school as trans facilities would be a burden on the construction in terms of cost or redesign.