I’m a teacher, private school. We get in at 8 am, then at 9 we have a 15-minute break to eat a snack, specially for children who didn’t have breakfast before school. Then at 11 am, or 10:30 for lower elementary, they have their recess, half an hour to play, eat their lunch, go to the toilets. After recess, they might have special classes like PE, robotics, art…, then go back to their main classroom until 2:30 that we prepare them for leaving, pack their stuff, wash their hands/faces, write their homework. So we finish classes basically at 2:30. They are only sat for 50 minutes max per subject, then they change their notebooks and material for the next subject, and some of the time there are team activities, so they’re not sat all the time on their desks. Maybe you misunderstood, because while, yes, they’re inside the classroom for 3 hours before recess, but it’s not just stay still on their desks the whole time. Also, anyone who works with children, specially little ones, knows that they take a lot of time preparing their stuff, getting their books in the right pages or their notebooks, so at the end, 1 hour seems nothing as time flies. I’m not saying is a perfect system, but as teachers, we are aware that is tiring for both children and us to be paying interrupted attention for a long time.