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.
Some of my classmates and friends began to wear black or grey long trousers in the forth year of the Prep School. With 2 elder sisters money was tight and a vest was a lot cheaper to buy.
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.
Agreed Tim, that was the whole point about wearing just shorts, we didn't need anything else, and didn't it feel great with the sweat running down! On the subject of which, after the first PE lesson, realising I had to go in the open communal showers I was embarrassed about taking my shorts off and being naked in front of the other boys, but I very soon overcame my inhibitions and took it all for granted. Felt embarrassed the first time but this was a boys' grammar school in the '60s and we had to get on with it. Depending on the amount of hot water available sometimes it was warm, but at other times bone-chillingly cold.
It helped that we were all treated the same: no vests, nothing under shorts and communal nude showers. We all had to strip and then walk under a shower in single file, as close as possible to the boy in front. But, whatever temperature it was, we had to have a proper shower using bars of soap, including washing our hair.