Teachers/staff prepare for those by knowing how to evacuate students from the schools and calling 911. They aren't expected to actually fight fires, aside from using a fire extinguisher.

"Readily available" can be addressed by concealed handguns that aren't too easy to get to; you might have to give an attacker a "first bite of the apple" in favor of making them too accessible. On the other hand, a few M4s in a quick to open safe in the principle's office ought to be doable, as long as the staffers there have the right attitudes, to "march towards the sound of canons".

In order for that to be effective and safe, those teachers/staffers will have to regularly train with those weapons in addition to their other duties. Look at the NYPD - we have a population (police) that are trained to use firearms and have to qualify at certain intervals and yet their rates of accidental firings were so high that they needed to have their handguns modified to reduce those accidents. Despite their training, their hitrate when they actually use their weapons on duty is abysmal.

Unless you have teachers/staff that are already firearms enthusiasts, they are probably going to be reluctant to add range time to their calendar, and putting a firearm in the hands of a poorly trained user will end badly.