I'm sure that women wore trousers because of work in wartime factories. I have old photos of my mom hiking (fashionably so, the men had carved walking sticks) in wide legged pants. Could that have been 30"s ? I have loads of pictures of her and her girlfriends wearing nice pants sets in the 40's, for leisure.
I graduated high school in '66. We wore pants everywhere, particularly those stretch pants with stirrups, that were a new thing. But not to school. The next year I came home
from college and the high school campus was a riot of pants, shorts, camisoles, halters.
Dress code completely out the window. I remember sitting in a college class and the professor making a remark about "women's lib" and the students, mostly guys, burst out laughing. I had no idea what that was. So I imagine that wearing pants to work was closer to the 70's.