There had been a lot of hype about this title on the internet, some claiming it to be the greatest horror manga of all time, so I thought I'd give it a read.
I think mostly what kept me interested in The Drifting Classroom was the sense of mystery attached to it. New elements keep coming into play in the wasteland where the school has been transported. While you're reading, you're never quite sure what will happen. At one point, a student's hallucinations become real, which could've been elaborated on across the characters. The concept disappears after a mini-story arc.
And I think that leads into one of my main problems with the series: it's too spastic. Everything has to be to the extreme, be it a character's reaction, or the quick resolution of a plot point, or that ALL of the teachers go crazy (conveniently), or the fact that 6th graders somehow know how to cure the black plague/do an appendicitis surgery. There's no subtlety to the story: there's quick conflicts always caused by unrealistically brash characters (who always die for this, such as when several first graders plummet to their deaths off the rooftop for wanting to "fly"), and then quick resolutions by a group of too knowledgeable students.
Things that happen in this story are too convenient. If it had spent more time with the kids NOT knowing what to do, it could've offered a deeper sense of dread. As it is, I was never really worried about these characters.