I keep seeing back-to-school sales on jeans and denim advertised.
A) Never was I allowed to go to school in jeans; even in public school it was against the dress code. So even in college, when I could have worn anything, I wore regular casual pants, not jeans. I didn’t like to waste my two or three pairs of jeans on non-fun.
B) August and September are too hot for jeans. Most schools have A/C these days, but buses and the outdoors don’t.
C) Probably kids wear jeans to school nowadays because they’re not allowed to play after school. It’s all sports and shorts.

It probably has to do with what you’re used to- our school didn’t have AC as far as I can remember, and I always wore jeans. No matter the weather, be it snowing or over 100. Half the time I was wearing a bulky sweater, too, but that was vanity.
I don’t think I OWNED slacks until my last two years….