There's some truly horrifying tales on here - not least the period diary stuff, which as a bloke I had no idea even existed.
I went to a state comp from 1987-1992, and as an overweight, self-conscious and bullied kid, I absolutely hated both PE and the associated showering. Luckily I had quite an understanding PE teacher who knew my limitations and never sought to humiliate me. The showering situation seemed to vary over the years - sometimes we were virtually compelled to do it, whilst at others teachers never seemed overly bothered.
In addition to the body confidence thing, the other issues for boys back then (and perhaps even for girls sometimes, I guess) was the scorn and occasional threat of violence you got if you were deemed to have glanced the wrong way in the communal showers. Homophobia was rife in the 80s and 90s, and any suggestion of gayness, or even a perceived lack of masculinity was met with aggression.
My worst shower experience was non-PE related, though. When I was about 12, we went on a school trip to a YHA hostel in Derbyshire. One evening, I went for a shower downstairs wrapped in just a towel, On returning to the dorm, there was a large groups of both lads and girls outside it. As I tried to make my way past, one of the girls wrenched my towel away. I was left stark naked and faced with a barrage of catcalls and jeers about my fatness and the paltry size of my 'equipment'. I fled into the dorm, utterly shamed and humiliated.
Later on, I told a teacher what had happened. He was a great guy, and a big influence on me (I still admire him to this day) but he pretty much laughed it off and encouraged me to do the same. I tried to, but the memory still hurts nearly 30 years later, and I think it's one of the things that really kill my body confidence.