Thanks for the translation! Hope you can finish it!
I feel like this is just the MC speedrunning ruining the teacher's life; tied to multiple assaults, soon-to-be murders, and he even preyed on a high schooler in the teacher's body.
I wonder why the 13th volume cover has all of them smiling together.
Well, this is revenge porn, a kind of fun guilty pleasure where consistency isn't paramount. Plot armour accounts for all. It's always like this in this kind of series, just check out
Revenge Classroom.
The final volume cover is probably ironic, a "how things could've been instead" cover. These sometimes happen in hardcore series.
But if we try to play devil's advocate just for laughs… in the case of Yui, it would be hard to implicate the teacher. It would be Yui's word against his, and the janitor could even testify that Yui went alone to the incinerator. The only thing the teacher did was stepping on his hand.
In the case of the girl… they're second-years, and the age of consent in Japan is 16, taking into account local ordinances against indecency. Teacher-student relationships aren't uncommon in manga, even non-romantic ones.
We shall now begin ethics has a female student confessing to the protagonist in the beginning, end of the first arc and again in the end of the first season. An episode in
Impossibility Defence has a student-teacher consensual couple. It's probably frowned upon, and kinda taboo, but not illegal. Of course, I'm not going into the morality of the thing.
Kiriko's case is a little weirder. He vanishes from the next covers (while Yui appears disfigured in the cover of a future volume), so I'm assuming he dies. Kurose wore a mask and inhaled helium to disguise his voice, and Kiriko is convinced that it's not the teacher, nor has any reason to believe it is, so even if he survives, it won't be easy to incriminate him even if he comes to suspect him.