Also, I'll admit I kinda assumed that the obligation by law to attend school until you're 18 is more or less universal, but I'm not sure how does it work in Japan, I think in quite a few places that could land the parents in more serious problems rather than just shame.
Compulsory education in Japan is only 9 years (i.e., through middle school).
Should be the case in most places. 9 years or until you're 18, whichever comes first.