I finally played through this game. Needless to say, it had some nice horror vibes.
As for the lore, There are a few things that make you think:
1. Why aren't there any other teachers and 2. Who hired the MC?

Like I said there is a story here they didn't bother to tell.

It definately shows. I wonder if...
In his youth the teacher, upset over rejection killed all of his classmates by burning down the school. The MC and facalty were the only survivors but despite getting away with it, MC was racked with guilt and his sanity suffered. Many years later, an adult MC just lost his job and under the stress his mind fully snaps and he imagines that he is a school teacher and that one of the letters re recieved (Perhaps junkmail) was a job offer for the school where his insanity began. He shows up to the condemed school where while being stressed, isolated, and exposed to that enviroment his insanity goes into a complete mental break. He imagines the school as it was in his youth and his classmates as they were while he was a student. Unlike in his youth though, the students are only the girls he remembers and are oversexed and sadistic. The sexuality reflects his youthful self's crushes while their sadism reflects both his sting of inrequited love as well as his guilt for killing them in the fire that he started out of despair and anger.

A naked adult man screaming and jerking off in a abandoned building for 4 hours until he snaps back and go home