I gotta say, I mostly disagree with everyone else's assessment on who's getting their rightfull comeuppance for sucking on earlier chapters. I don't think anyone in these chapters deserves to get screwed over except Rinko/Riko/The bob hair girl, and another character I'll name later on in this comment.
This manga is very melodramatic, but it is representing how previous mental harm manifests at the faintest touch, be it an insult or perceived behavior, benign as it may be, the reflects upon the aspects of our past and personality we'd rather forget. All these characters lash out on each other on the account of deep-seated issues that, as teenagers, they haven't had the chance to properly address. Not saying they're not wrong when they do so, but that they aren't doing it due to a callous personality or ill-intent, it's just the recourse of action they think they have to protect their damaged psyches.
Again, except for that Rinko tart, she does deserve to get screwed over.
All this being said, there are two other characters who stand out from this description, which are Ichikawa and that blond boy.
Ichikawa got GANG-RAPED for doing superficial bullying that seemed awful in the earlier chapters, but is comparatively mild in light of what every other girl in that gang has been shown to have done in the chapters since, except maybe Ruka.
For all we know she's still attending school, the last place she was seen, and not only have all the other characters moved on without taking any further notice of her, but the story seems to just have dropped her, after suffering the worst violence displayed in this manga so far.
Her sexual assault has not only been dropped as a plot-point, but it seems like it doesn't even reflect upon or is remembered by Kanae. That's the worst and most important thing she's done, and she's forgotten about it, and so far it seems like the author has as well, and wants us to too.
Kanae just casually set up a rape sesh, no biggie. She deserves a Crimson Chin level of fist-delivered justice, yet the manga just has us forget about it, and continues to portray her failings as lying elsewhere. Hell, the plot empathizes with her as often as it doesn't, the author is trying to ingratiate us with her and build up sympathy while she already crossed the moral event horizon over 50 chapters ago, in a story that treated this subject with due seriousness, she'd be irredeemable.
The blond-boy's also in prison without any continuation to his role in the story. At least some readers can take comfort in knowing that he's jailed, whereas no redress or resolution has come for Ichikawa's place in the story, but it's still a lackluster, inconclusive and trite ending.
I don't know if the editor compelled the author to drop the sexual assault shenanigans due to commercial concerns, or if the author just used it as shock value, or just got tired of it, but as this has been a solid manga, if not a little histrionic, the way rape has been treated as forgettable and unworthy of care, forgiveness, healing, or anything at all, is disappointing.
7/10, would recommend.
Edit - Read a bit of the rest of the author's work, whis is all hentai, and I realized that they revel in human misery, fetishistically so. Shouldn't have expect someone like this to really develop upon the nuances of great sexual pain and suffering.
It would beggar belief that this manga managed to strike some emotional chords when penned by this kind of person, but if we put two and two together from the chapter 21 afterword, we can understand that a lot of what these characters are doing, thinking and going through likely relay from the author's own livelihood and experience, not from a study of fellow human behavior or anything empathetic like that.
Edit 2 - Looked the author up on Namu Wiki, got confirmation that indeed these dark themes are based from the author's own personal experience, but also that she's a woman. A woman who sexually enjoys drawing little girls being kidnapped and raped, and doesn't particularly care about sexual assault. Damn dude, no solidarity with her own gender's worst ailments.