I am writing this from a UK Schools' perspective in the 1960s (but I am sure it probably applies to schools at the time in the USA, France, Germany or indeed anywhere in Europe or the ROTW).
In common with many, I am sure, there was a lot of 'naturally occurring nudity' in those days and boyish camaraderie ( as well as risky - in terms of being punished with CP if caught - stupidity / horseplay etc). We had PE / Games twice per week with mandatory showers. I was in the school Cross Country team....so that included additional training / competitions etc....all of which also involved having to strip naked to shower. We also had swimming lessons (ditto - although we wore speedos for the actual lesson time).
Everyone of that age / era will recall a lot of nudity whilst changing / showering etc. You got used to it, up to a point. Of course, there were always 'poseurs' who flaunted their naked bodies. Others just got on with it, without being shy. Some boys were modest, others were prudish. We all knew the unwritten rules and (historical)....boys simply dared not behave in any way which might see them accused of being 'gay' or 'queer'. We checked each other out discreetly. All part of boyhood adolescence.
There were occasions when nudity caused humiliation and embarrassment though. I will relate an incident in my next post/comment to show what I mean. Maybe it will evoke similar memories from others.