gymslip

True story:
Before the 60s, schoolgirls were expected to dresses or skirts at all time - regardless of the weather.

In some places, girls were forbidden to go to school in jeans even if their families were too hard-up to provide skirts.

What Goodgulf says is correct. Not just schoolgirls but slacks on women were frowned upon in the workplace and many social settings until the 60s or even the 70s.
One of the first places pants on women became accepted was in hospitals. Nurses found pantsuits made moving around while dealing with patients much easier than in skirts with their constriction, not to mention not having to worry about hemlines rising up to unseemly levels.
All in all, the 60s brought about a great freedom of choice in dress for both men and women. Hard to realize that once upon a time men wore suits and ties almost everywhere, to ball games and movies, not just to work in the office or church.

There's a reason they use to have burlap skirts - that is skirts and dresses made from old burlap sacks. Wearing thread worn dresses was seen as better than allowing them to wear slacks or jeans, even if they couldn't afford dresses.

Then there was the other side of things. "Proper" people dressed properly - which often meant layers of clothing in the hottest weather. For example, wearing a three piece suit (and suitable undergarments) in the hottest part of the day in India.

All the girls in "The Belles of St. Trinian's" wore gymslips.

In my schooldays, back in the '70s, the girls in my school were allowed to wear jeans - but only when the temperature dropped below a certain figure, as measured by the thermometer on the outside of the girls' changing rooms.

One could sometimes see them surreptitiously holding a bag of ice-cubes, acquired from the school kitchens, to the thermometer in order to drive the temperature down to the required mark...!

Here when the Catholic school girls are allowed to wear slacks due to cold weather, it is never jeans. It's basically khakis or other "semi-dressy" pants. Generally specific ones permitted by the school for winter uniform.

Remember a few years ago when there was a movement advocating uniforms for American school children attending public schools. Supposedly it would solve all the problems of our education system(just like school prayer would). Don't hear much about it anymore. True, other nations do require school uniforms, but we Americans like to pride ourselves on not doing things other countries do. Sometimes thats a great idea, sometimes not( like when we stubbornly refuse to provide a system of healthcare for all.)

For once I'm with you yanks all the way, no school uniforms here ever.

I personally don't have issues with sensible/practical school uniforms, especially with the changing fashions of youth, yes we wanted to follow the fashions but on reflection they were not entirely conducive to school activities.

My gripe is the cost of them, as so many are in very specific colours or mixture of, wheras if the uniform was in regular off the peg colours eg black then cost would be manageable, but regrettably they are not, often you have to buy it from the school or specific outfitters who stock it.