[Windflier]

Probably the easiest, quickest, and most effective thing that schools can do, is to take down the "gun-free zone" signs, and replace them with signs saying,

WARNING: Adults on this campus are armed, and will use deadly force in case of threat to life.

Spread the word through internal announcements and news media.

This can be done before actually arming any teachers or administrators, and will act as an immediate deterrent against would-be assailants.

[stuartcr]

That would be probably the most effective and cheapest method to achieve quick results.