Probably will be pretty long. The last time had a couple of days before the murder was done, right?
Spoiler:
The worst incident in the history of humankind? So it's kind of apocalypse?
I think that explosion that the fortune teller heard when he got near the door kinda hint something like that...or perhaps, a great war or some kind of giant anarchy?
If all of this is because of the headmaster, why would he dot that if the situation outside is as what we predicted? Is he trying to select his student by making them killing each other like this? What merit would he got by doing this?