Hi Claire, at least you were all girls and presumably with all female teachers during PE.
In our case it was mixed and with both male and female teachers, depending on who your class teacher was.
There are other ways to distinguish teams but none as effective or practical than stripping off.
Boys realise that there's no hiding place when you're stripped to the waist. Certainly I noticed especially during the Summer Terms at my mixed Grammar School when we did athletics outdoors many of the girls would gravitate to the boys who were stripped to the waist - nearly all of us - like bees around a honeypot. I think the same split would apply if boys were given the choice, even if only for outdoor PE.
The governors were all retired ex-military types anyway who no doubt thought the more severe a caning the better - never done them any harm and so on.
The headmaster maintained a list of boys who he saw as rebellious and after the skinhead cut I was on it alongside a few more.
It guaranteed you a bare bottom caning if you were sent to him and up to twelve strokes - which I never got though I did get eight a couple of times - surprisingly much worse than six.
I doubt any parent today would use a strap like my father had either - it was about eighteen inches long and two wide and half an inch thick, it really packed a punch and a dozen of it on my bare bottom was the norm.
I guess most of us started to feel embarrassed being half naked or almost naked in front of the opposite sex at that age.
We all undressed down to our underpants in class under the supervision of our teacher and were taken for PE exercises. I wonder if this is still permitted in any schools.
As you say, some of the girls started to develop budding breasts during the final year at Primary and a few started to wear bras rather than vests.
They were normally allowed to change separately in the toilets.
Peter B, Of course these days the cane is banned so it just couldn't happen but back in the 1960s a headmaster, particularly of a private school had pretty free reign where anything was concerned.