Juniors and seniors who are college bound typically add 3-4 hours of cram school in the evenings to prep for college admissions tests.
Death by overwork is called karoshi. Japanese salarymen will often stay at work until 10pm or so. Then it is expected of them to hit the bars with their co-workers until midnight, then come right back to work at 8am the next day.
For a female to die that way is rather unusual, as the large majority of salarymen are male.