I was at the classroom a few times while class was in session. The kids weren't out of control. They paid attention during lessons, but they weren't 'corrected' for standing up, fidgeting, or whispering to their neighbor. What 'assignments' they did in class were always done in groups and the groups could get as loud as they wanted. Nap time was often turned into extra playtime.
They let the teacher go at the end of the year. For first grade, the two classes were reshuffled, so as to mix the 'rowdy' kids in with the good ones.
Funny thing was, as a group, the rowdy kids had already covered the first quarter or so of 1st grade. Their grades in first grade were consistently better. According to a second grade teacher there we knew well, it wasn't until about midway through second grade that the original quiet kids caught up to the original rowdy ones as a group.
Did this experience change anything? Nope. My daughter's now a senior in high school at an entirely different school system, and both there and at her original kindergarten, it's still sit down and shut up.
Did you just make that up or is there evidence that this is actually the case?
Personally, I would have a hard time paying attention in meetings and lectures if most of the people in the room were constantly getting up, sitting down, playing with stuff, answering their phones, etc. Do you think maybe we ask kids to sit quietly because it's distracting for everyone else when they don't?
And it is true that some task require quiet. Math is one of those tasks. But I as an adult have no need to fidget all that much.
And if I remember correctly, what you learn in the early grades doesn't need that much sensory isolation, I image things quiet down during exams.
So I think the degree of stillness can be slowly increased until you're expected to sit completely still in college.
The whole sit still for hours at a time, is unnatural for kids and comes from the time when school was in a monastery and the monks thought fidgeting was devil's work.
It is amazing how long counter productive rituals can persists. Requiring kids to sit eerily still, makes teaching them harder, for no good reason. And it has been that way for hundreds of years. Civilization can be seriously crazy some times.