Part of why this works is because Japanese school is structured differently from American school. In the US the class of students moves between teachers’ rooms; in Japan the teachers move between classrooms. So things like lunchroom coats that fit the particular students are always in the same place, and the lunch is served in the classroom so you’re only serving a few dozen people.
And in moderation, so children have lots of unstructured time to play and learn and socialize on their own.