There are several places in here where you come across as very transphobic. If you don't feel comfortable changing in the same room as someone who is trans, then you should be the one who goes somewhere else. I feel like most of the time though, you'll probably have no idea who is trans, and there's even a chance you've already changed in the same area as someone who is trans and you had no idea.
I too took gym class from 1995-2009 in K-12 schools. Had to change for gym starting in 7th grade. Never saw a single person naked in the locker room. People would be in their underwear, but there was never any reason for people to be naked. When we did have to shower, after swimming in gym, the showers were all individual and each had their own dressing area. I'm sure in schools it was different.
Telling people to just visit a Japanese hotspring and "get over it" is extremely insensitive, but I'm not going to bother trying to talk to you more because I remember trying talking to you before here in a different thread and it was pointless.