There was a 5-10 min window for teachers to walk from class to class. So yeah we were there the entire time, 8AM-3PM. Same thing exists in the lower grades in US, K-5. We don’t see those kids complaining either.

Frankly, that sounds awful to me, even as an adult; it sounds like a recipe for sedentarism and vitamin D deficiency. In my country we have 15m recesses between each class, starting from 5th grade, where we got some sunlight and exercise - at least climbing the stairs back to the classroom, but many kids played some sport or game with physical activity.