I've been thinking about where I would carry a firearm in a classroom to keep it safe from curious kindergarteners or angry teenagers. Unloaded and locked in my desk drawer with a trigger lock, and the ammunition locked in a different drawer is one possibility though not exactly an instant-response opportunity.
Alternatively, a loaded pistol in a bra holster could provide quick response while providing an impregnable resting place. Fortunately, I have two advantages: (1) a small bust, so that the extra padding would not be overwhelming, and (2) left-handedness, so that if the gun were to go off accidentally (actually a more likely occurrence than shooting a terrorist) I wouldn't shoot myself in the heart.
Either way, I can't imagine sending a child to a school where the teachers carry weapons.