@trapsarebetter
Legally, Japanese only requires people to attend school through the end of middle school. High school is technically completely optional, although the vast majority of people do attend.
With that in mind, it’s probably up to how much the school cares about their image - I’d imagine having students who don’t attend class would probably cause trouble when their scores on standardized tests are accordingly low so they’d have something to gain by kicking her out, but on the other hand they’re probably getting tuition money for her without having to do anything in return and so might be willing to keep her enrolled regardless of whether or not she actually shows up.