The number of classes that Japanese students attend per day are about
4 for early primary (only about 40-45 minutes each) and about 5 for
late primary and middle school and above (about 50 minutes each at
middle school and above), in some cases 6.
That looks very similar to US classes, but I grant you, an hour less. Although our primary schools generally use a single teacher. I wonder how Japan schedules the teachers. Are you saying that they have twice as many teachers as us? It just seems hard to believe that Japanese teachers only teach 2 or 3 hours a day, albeit for many more days in the year than US teachers.