They would see real fighting and ran the risk of injury or even death so the teachers thought they needed to be toughened up. Decades ago the law permitted teachers to dish out punishments no Prisoner would receive under law!
I now realise one of my PE teachers was probably suffering from undiagnosed PTSD from the War years previous.
Then when I was 14 it all changed, we were put into sets instead of classes for PE, which meant larger groups of boys and vests against skins teams.
I remember feeling so awkward and uncomfortable the first time I was told to take my vest off, like you I wasn't used to that.
Turned out he had a birthmark on his chest that he was embarrassed about.
We used coloured bibs to differentiate between teams.
We had shower cubicles in which we showered in private. As you say by the age of 14 the shorts did not cover much.
I think it was due to not being on the uniform list which stipulated a white PE vest.
Despite bringing an ordinary vest for the next lesson the lad was kept in the skins team for the whole term. I only had a vest on literally a handful of times during all my years at school.