So, it literally ends in the middle of the event. They didn't get to the 2nd round. Was the manga cancelled? Because that does not feel like an ending of a series, much rather a cliffhanger for another volume. 

There still is absolutely no world-building explained and I haven't even seen a teacher. Makes me wonder if that whole event is just a student thing, and the teachers are completely unaware of that. Might also explain why there were 20+ people on a high tower, with absolutely no safety measures. Nagisa was about to fall to her death even, had someone not rescued her.
Seriously, what kinda school is that? No, what world is that? I'm just confused.

Maybe, if I'll ever feel like I need some answers to this, I might check out the anime, but other than that, probably not.