Why do we make children sit still in class?

As a graduate student, I had the opportunity to observe a number of idyllic, progressive classrooms where students danced to the pencil sharpener or sprawled across beanbag chairs while completing their work. I read countless books and articles about research that supports physical activity as part of academic success. It made sense to me -- theoretically -- that children should be allowed to move their bodies. Asking them to do otherwise, I came to believe, could be detrimental to both the student and the teacher.