My 11th grade history teacher was giving our class a tour of the “catacombs” of our school. All of sudden, he said, “Back up! Back up!” We had reached the corridor that surrounded the school pool, and it had little windows in the pool’s walls. We asked him why we had to back up? “There’s a boy’s swim class in there, and they swim naked”, he explained. I don’t remember if I already knew that or not, but yeah, it’s true. He said it was for cleanliness.
This was 1968, btw.
When I was in high school we had to shower after gym, naked of course. This was during the 60’s and had been the schools practice for a long time. The only drawback I saw was if you were a late bloomer, you were probably picked on. I saw a few boys in tears. For the girls it was optional, whether to shower or not.