This... Is fundamentally bad writing.
Bad writing for a mystery. Like, classically bad writing for a mystery. It's okay as a different genre.
We have had eleven chapters to develop characters and world settings. That may not seem like a lot, but it is still three volumes, and out of 21 chapters in total, this is more than the halfway point.
That means the author had ample opportunity to flesh out characters and build up a list of possible suspects. Which the author kind of did. With the girls.
But then we have literally the first named boy in this entire series dropped onto our laps in the eleventh chapter. I get it. This isn't the actual big bad guy. He explicitly says it. But my point is that the author should've done SOMETHING to even introduce the character before this point.
The author could've just had a quick page in one of the first chapters where it introduced Akane to the class, and this boy would be one name out of the many in the classroom. That would've been significantly better than this crap where the author can just invent this character in this chapter, without any forethought or planning.
It's bad mystery writing specifically. A mystery should at least kind of sort of hint at the reveal. If it doesn't, then it's just deus ex machina writing.