I certainly remember P.E. classes where the boys were in dark blue shorts and white tops, with the girls just stripped down to their underwear. As mentioned above, it was pretty common for the boys to be the same if they'd forgotten their kit. Later on the girls had shorts or a short skirt, depending upon the activity.
at my sons Junior school a few years ago when they had the kids in year 6 all getting dressed for P.E. all together in the classroom
I've lost track of this new numbering system somewhat. Year 6 would be what, the last but one year of junior school? In the 1970s/early 1980s we used completely separate numbering for infants, juniors, and high school, so we went from 4th year juniors to 1st year high.
I certainly remember the whole class getting changed together in our classroom before marching down to the hall/gym, right up to last year of junior school (age 10/11). That was in 1977.