Yuki leaves home to go live in a dorm when starting junior high at 12 years of age.
Is this normal in Japan? Does kids normally leave home this early? Or is it just those who live far away from school?
I don't think that's the norm in Japan, no. But I believe it was mentioned that the nearest junior high to Hana'a house was a 2-and-a-half hour drive, so seeing as the school had dorms, it probably served lots of families who lived in remote/isolated areas and couldn't drive their children to school every day.
shareDorms are less common in Japan than they are in America. Students often commute 1-2 hours each way to get to class. The only people that dorm are usually people that live even farther away AND happen to go to a school that actually has much in the way of dormitories. Otherwise they get single room apartments (room sharing is very uncommon). Very few schools in the biggest cities have very much land at all, and mainly build upwards instead of outwards.
I don't know much about the high school system, but it seems that overall there's a much stronger force toward staying at home and commuting long distances, as opposed to a dorm system. Most high school students live fairly close, but a significant number do go quite a distance, which leads me to believe that dorm's are more common in rural areas.
Public high schools also have fairly significant fees, so that's often an incentive to getting into a good private high school, which could include a significant commute... or in the case of some, living away from their family even at a younger age. Perhaps kind of like an academy.
It's been a while since I saw the film, but as far as I know, Yuki wasn't in a private school and as lagrammairienne said, Hana's house was over 2 hours from the school, perhaps this distance and the fact that Hana isn't exactly financially well-off contributes to Yuki living at the school.
Why is the Wolf always bad?
http://www.fourpastmidnight.blogspot.com/