Even if this wasn't just another story filled with misogynist tropes of helpless dark-haired girls, the cunning blondes and dark-haired dying young from diseases that apparently only kill young dark-haired mothers, it has plotholes and internal inconsistencies all over the place.
The mangaka couldn't even be bothered to have things somewhat plausible when it comes to weaponry. Having the girls run around with crossbows, handguns, and grenades of all things instead of having them fight with what can naturally be found in a school building.