My 10 yo DS wears jeans to school almost all the time. He does have a preference for a softer, slightly elastic jeans vs hard ones (you can find both types at Gap, JCrew, or similar stores), which is what I think people are commenting on. DS is very active and never complained about not being able to move freely while wearing jeans (even in the summer). His jeans probably saved his knees on several occasions (tore through jeans, skin barely scraped).
My other kid used to like jeans, but now sticks to soft cotton chinos. I like the polished look of both, but it is their decision in the end (I know I won’t have much say for too long

).