ETA: Perhaps this is different in different parts of the country, but my peers and I (Millennials) almost never got naked or used a public shower in our high school locker rooms. That is largely antiquated in my experience. Showers were technically available but never used, and quite gross and ancient. We would strip down to our underwear to change in PE but that's it. Perhaps other people my age or younger have had different experiences.
I remember the showering rule in P.E. changed while I was in high school, late 70s/early 80s.
At some schools I went, it felt like P.E. class was structured to prepare kids for the military, at least on some level.
In Starship Troopers, they showed men and women soldiers showering together like it was no big deal. I don't know if society is ready for that yet.