Since there's a video-game background looming around, it could be elf-sensei was playing said game prior to the collective death of that classroom ?
Also, numbers were only stating that one classroom. But since the incident felt like a huge explosion, or at least a wide area/volume catastrophe, the whole school body is dead I guess.
Would it be possible there's a comp beetween classrooms ? Former teachers being captains ? Or if not a comp, teachers have meetings and think strategies to find their students, protect them in that new world and groom them so that they can survive longer ?