Iijima's killer is neither Maria, nor Okaya's bully group. Huh. A third party, interesting. I'm very surprised Okaya was not involved.
Based on my list of characters, candidates who are not Maria, the bullies, or the police, turn up two names: Yuda, a male bullying victim and Yajima, Kowase's rape victim. I'd probably rule out Yajima since it's less likely for a broken rape victim to be the one to murder a violent young adult man by herself. If it's a completely new character that would be a bit surprising, but not so much considering Iijima has his fair share of enemies from all the badger game scams he's done with Shikimi.
Sad to see traces of the old Mari surfacing in Maria from time to time. But that also shows she's not yet fully become the personification of vengeance that she wants to be. She's dangerous, but not infallible or invulnerable. It's also annoying how she keeps indulging the sleazy vice principal, but as long as he has the power to keep or kick her from the school, I don't think we'll see him getting destroyed any time soon. And until her vengeance is complete, she needs to stay in the school to make it easier to keep killing her son's killers.