From what I recall: Jimi (as naive newb teacher) was in love with older sister (unrequited). He then got a job at the school where the younger sister was. Older sister knew exactly what sort of monster younger sister was, but didn't give any warning to Jimi, who then got royally fucked by younger sister's stunts. Then, after Jimi was recovering in the hospital (from a suicide attempt?), he gets visited by older sister, who was now getting married and moving up in life. The unfairness and betrayal of her behavior completely pissed him off, so he blackmailed her to assist him in continuing to get work at a teacher (because otherwise he was out of work due to the wreck that the younger sister made of his life and career).
The core of the story is: The damage done by evil people in power who can and do destroy other people just for kicks. Secondarily, the degree to which other people fail to step in and stop or at least partially alleviate that damage. E.g. the younger sister (Jimi's former student) who is one of the evil ones; the older sister (principal) who did nothing to help Jimi. Vs Jimi who is a barely functional wreck still manages to help somewhat, though he adheres super strongly to his "just a teacher" boundaries.
And though the powerful at the main source of evil, the rest of the world is not short of lesser varieties of evil: the compensated dating old man who wants to force sex on the last date; the dad who's into his daughters school friends; the fellow teacher who plays off his students attraction to him to manipulate them; the reporter who just wants to dig up dirt regardless of who it hurts.