From what I’ve heard – and it’s hard to get any reliable information on this from the western internet – school hours in China are so long partly because they’re broken up (as in Cuba) with exercise, meals, and rest times. Also some kind of mandatory study session. And the system, while it may very well need some kind of reform, does seem to work; people talk about “socialization,” but American schools are not exactly doing a great job of that either. The Chinese system, at the very least, seems to produce educated individuals and no mass shooters, while the US system fails on both counts.

I would guess that 12 hours is probably an extreme case, since the fact that the government needed to implement measures (parental sign-in, and so on) to combat video game addiction among minors argues that kids do, in fact, have a lot of free time.