At Crossfields, an all boys public school, swimming lessons involved compulsory nudity.
I wish this wasn't true! The practice ended just after I left, due to the 'self-consciousness of the boys'. No shit. This was in 1985.
A nice twist came when the swimming teacher's daughters (in cossies) were in the pool on some sort of open day thing, and we all dived in to join them, much to their embarrassment, but not ours.

It was only reading the previous story that it struck me as odd that at my primary school swimming pool, the boys changing rooms had one wall which consisted of a huge, sliding patio-type window. Facing onto the school field. And swimming lessons used to regularly run over into breaktime, when, of course, the whole school was allowed, en masse, onto the playing field. Which, to add insult to injury, was bordered all round by houses.
To this day i cannot think of one possible reason as to why changing rooms in a swimming pool would be required to be glass-sided, aside from the obvious solution that perverts were involved at every level of the design process.
There were also 'helpers', one of whom was my grandfather. Part of the 'helping' remit was to ensure that boys got changed properly and promptly. So any boys resisting the public change would be virtually wrestled into the room, and forcibly dressed in front of the school, and town, by my grandad. Most opted for the easier path.