I lived in Japan as well. There is an hour in the morning for music or something and an hour or two for after school sports. The actual in class time wasn't that bad. Plus a lot of time spent socializing and hanging out. Some kids would be out till 8-9 before they came home. They weren't in class all day.
They do better because they have a culture that encourages it.
To downgrade the human mind is bad theology. - C. K. Chesterton