They were what girls wore to gym class until the mid-seventies in the US. This stopped after coed gym classes became the norm in public schools (before then, before Title XI that tried (and failed) to ensure equal funding/status to women's sports, boys and girls did not play kickball together after elementary school.)
I went to two junior high schools (NOT middle schools, not intermediate schools. Junior high schools.) The first, Cunningham Junior High, is in Brooklyn. It went from 7th to 9th grade (high school went from 10th to 12th.) And it was my first (and last) year with official single-sex gym class. In elementary school, when we started gym in second grade with my beloved Mr. Biegel, who was kind and patient and never seemed to mind I had no athletic ability at all, we went as a class. Also, we didn't change - we just wore sneakers. This being 1970 or so, we had bring them because no one wore sneakers to school, and most little girls wore dresses. This changed in only two or three years.
Cunningham did not have lockers of any sort. We went to homeroom (as a class - we went to all our classes *as a class*, other than shop and an elective, but we all had shop and elective at the same time) four times a day because our coats and books would be locked in a classroom wardrobe. We put them away in the morning, retrieved them for lunch - I went home for lunch - put them away afterwards and got them at the close of the day. As for gym class - we changed *in the gym*. In the girls' gym, I mean. We had gym twice a week, and we were required to wear gymsuits.
I don't mean gym clothes. I don't mean an official t-shirt and shorts - the *boys* got those. I mean a gymsuit. This was a navy blue one piece garment that snapped down the front, made of some woven material - probably at that point cotton-poly, but possibly cotton. The top looked like a short-sleeved blouse with a breast pocket and a regular collar. The bottom, which was separated by an attached "belt", looked like a pair of short bloomers, full and gathered at the bottom.
Yep, as ugly and uncomfortable as you can imagine, and I suspect those of you who are my age and older are having flashbacks now. (Although I do know some girls liked them.)
We were instructed as to how to change with minimal exposure (take off pants - we were mostly wearing pants/jeans at that point), put bottoms on. Take off top, put the top on. Snap.) We hung our clothes, book bags and the purses we were all starting to carry on hooks over our assigned bench - gym class was four classes at a time, and we each had our assigned bench against a wall.
We did have one advantage over the boys - when weather permitted, we could have gym outside. Since no one expected us to wear those ugly things in public, we could have gym outside in our regular clothes. Boys had to wear their gym clothes, so they had to wait for warmer weather.
I moved to Wayne, New Jersey in the summer of 1976, where I started 8th grade in Schuyler-Colfax Jr. High (for the presidential scholars among you, this was named after two men - hence the hyphen - not the vice president under Grant.) This was 6-8 school, so at a time when one went to junior high for three years, I went for two. I had to get used to lockers and combination locks,and having my own schedule that was the same EVERY DAY and having lunch in school and not having Spanish, and, of course, being the new girl. This is where I learned that having a book to read during lunch made everything better.
And this was also the first year of co-ed gym. Since it was a transitional year, they still required the girls to wear, you guessed it. Gymsuits. These were a different - still ugly - but far more functional model. It zipped up the front instead of snapped, and it was knit, not woven. The top part had thin maroon and white horizontal stripes; the bottoms were close-fitting maroon shorts. I thought that co-ed gym was normal for New Jersey until a classmate of mine, who was having health - classroom work - that first marking period, asked me how it was having boys in gym. And then I saw a news article - the gym teachers were still trying to figure it all out.
By the next year, everyone was wearing shorts and t-shirts and the era of gymsuits, at least in Wayne, came to an end.
ETA Pictures. The one on the bottom is the closest to what I wore.