"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".
And it's attitude above all that needs to be addressed, we don't take this threat seriously, perhaps in part because it's quite new as these things go (we'll ignore the far more deadly 1927 Bath school massacre done with explosives).
This school had a crust defense; once the perimeter was compromised by the attacker shooting out a window (according to the latest never very reliable reports) there was nothing left but for the teachers to interpose their bodies between the shooter and their charges. Which was no obstacle to him (one does wonder what it takes for someone who doesn't seem to have suffered from schizophrenia or mania to shoot a bunch of 5-6 year old children 3-11 times each ... I admit the existence of evil, won't claim to ever really understand it).