This was a lot of fun (in a demented way, of course), and I found myself marveling at the author's plotting (figuring out pacing, how/when to kill off 42 students, each one's little story, etc.).
I was a little annoyed by all the lead-up to Shinji's plan that didn't even get executed, and I know that was part of the point, but for nothing really to have been done with that (besides trying to blow up The Terminator, i.e. Kazuo)? I'm glad it was Noriko in the end who shot Kazuo because I was getting sick of her just being The Girl, perpetually wounded and supporting the other guys and mooning over Shuya (like practically every other girl, jeeze). Especially when there were other kick-ass girls (my favorite being Takako, track star), even if one was pretty crazy (Mitsuko).
The various weapons, fights, interactions and relationships between the students were inventive, and there were constant surprises. Like I said, a lot of demented fun, and also moments of real sadness (e.g. "No, not THAT person!").