Replying...
Intro. You were sitting in the classroom when you saw from the gate a man standing in front of the school, then the school guard asked, then the guard was bitten by that man, after being bitten became a zombie. The teachers were trying to stop at the school gate, you saw that something was wrong, so you ran to the rooftop and saw that the whole city was full of black smoke.

Zombie School

@Shikanagi Yuminaga