UK view here.
Gym or PE as we call it here, is compulsory up to 16. I enjoyed it (sometimes) but then I'm a country boy. Hated the changing rooms as I was quite shy. (not any more as an adult)
Sometimes the teachers would let us do what we liked - football, rugby, cricket, other times they would test out some weird notion of the day e.g. non-competitive circuit training. (hey, I understand it now, but back then, it was weird). Didn't like the running stuff.
Every now and then, they would try to enforce showers, but everyone complained. I hated them too. They got less keen on enforcing showers as we got older, so in my last few years, it was always optional.
Countryside small town school - everyone walked or cycled to school, from age 11 onwards. We had maybe 1 or 2 fat guys in the whole school.
Usually boys and girls had separate gym classes. We had combined classes once in a blue moon, maybe when staff were off. Looking back, some more combined classes would have been nice, e.g. playing rounders together.
Our PE staff were weird.
The senior PE teacher communicated in grunts, and never spoke to me once in 5 years. Once, he took us to a weightlifting club in town, and us gangly kids worked out on the machines, and he spent the whole time staring at a well built (adult) female bodybuilder in the mirror, who was on some sort of chest expansion machine, trying to hide the turmoil in his trousers while telling us not to stare at her
The other younger PE teacher had a flash 'tache, and drove a sports car and fancied himself a ladies man. 15 years after leaving the school, (and moving to London) I came back to visit family, and saw in the local rag he was serving time for shagging a student. (it seemed to be a consensual relationship, but she was underage and he was married with kids - oops.)
The female PE teacher was also weird, she was about 40, tough as old boots, very sporty, and had this habit of wearing her tight gym trousers hoisted up VERY high so that you could see every detail of her front bottom. This was much commented on by all us kids, both girls and boys.
Another popular topic post-changing rooms, for both boys and girls, was who was the most hairy, with some people proudly sporting small forests down there, and others with just bum fluff.
I'll leave USA MR people with the thought that while in the UK, we had segregated changing rooms, in France, and I think some other EU countries, school changing rooms are communal - much to the shock of some of my friends who went on exchange visits. Toilets too, often with no doors
