@rel_ it isn't clear yet. But at least we're getting glimpses. So far what we know for sure are:
-he had a student that was bullied, ostracized, and gangraped.. which lead her to committing suicide
-the sister of that student was actually the chairman of his current school
-he said that he didn't like his job, yet he's still doing it (which makes me think that he's doing it because of the chairman)
-since he's being "tied up" by the chairman, it makes us think "why does she even let him work as a teacher, if he failed as a teacher by not saving her sister"
-based on the last point, it shows that we still don't know exactly what happened 10 years ago
thus why I think the story is actually good.. or maybe I'm biased coz I like psychological genre