This may be off-topic, but I thought I'd bring it up: in a nut shell, defensive "use once" less than lethal weapons for public areas. They would be strictly licensed and regulated by first responders, much like public defibrillators are.

While I found NRA's idea of arming teachers to be (let's be honest here, and I say this a strong second amendment supporter) absolutely nuts -- several people have circulated the idea of providing less than lethal weapons to teachers and administrators. I think most teachers (or most people in general) are not keen in owning a firearm, do not have the time to go through the training to use a firearm effectively in a high-stress scenario.

However (and this idea isn't original to me, I've seen it suggested elsewhere online) provide a modified than lethal weapon (e.g., a carbine length taser) in each classroom -- hidden behind glass door much like a fire extinguisher or a defibrillator would be. They would be given training in using this weapon to stop (or slow down) an opponent and there would be strict rules to ensure it cannot be used for any other purpose (e.g., it would have "drive-stun" capability removed and be limited to only a few rounds). Shattering the glass in any classroom would immediately set of alarms in all classrooms (giving other teachers time guide children to safety) and cause first responders to come (irrespective of time or day).

While mass shootings do not represent most of gun violence, they are especially unnerving. Generally, however:

1) Mass shootings are usually murder-suicide. Suicide here is either a primary (with murder being secondary) goal or a way of escaping retribution. If, on the other hand, the perpetrator knows they are more likely to be simply disabled and then arrested and thrown in prison, this creates further deterrence: it now makes more sense not to go through with the plan, to surrender right away before committing any violence.

Sentencing guidelines could reflect it: attempted school shooters who surrendered without firing a shot would receiving more lenient sentencing (but the case itself would be sealed, put on a gag order to prevent those seeking notoriety from making attempts), those are arrested by force would receive far stricter sentencing than those surrender voluntarily (idea being surrender voluntarily/commit no further crime crime < captured by force/commit no further crime < surrender voluntarily/commit further crimes < surrender by force/commit further crime).

Essentially the goal would be to sent two messages:

I) If you are suicidal, you're far more likely to fail, be captured, and have your life made much worse (on top of what ever is ailing you) if you try to "take others with you"

II) It is very difficult to escape retribution in a mass shooting, so the best strategy would be to either not attempt a mass shooting or to peacefully surrender without firing a shot.

2) Contrary to popular belief, mass shootings are not always in explici "gun free" zones (Giffords shooting, Portland Mall shooting, possibly the Aurora shooting) -- and usually a single armed guard or a CCW license holder might be there but wasn't be able to do much.

However, several shootings have been ended early by multiple unarmed individuals tackling a disoriented perpetrator. Obviously it is not expected for elementary school teachers to be able to tackle an assailant, yet this approach has the advantage that now there are multiple individuals (teachers in different classrooms) armed with less than lethal (which by no means means "non-lethal") tools that significant amplify their own physical ability and can disorient the assailant even without directly hitting the assailant (i.e., one volunteer using the weapon now makes the assailant more susceptible to additional uses of the weapon).

3) The less than lethal weapon should be designed with the purpose of making an otherwise untrained individual (with no firearms experience) not only able to incapacitate an assailant, but to also make them feel confident that they are able to.

That is why I think a "carbine/shotgun-length taser" might be better approach here than a hand-held tool: it would be easier to aim, look like a more menacing weapon, and fit a wider variety of individuals.

4) (Added this later) Teachers, guards, other volunteers have a "homeground advantage here" vis. an intruder. This would be more effective than a passer-by CCW holder in a mall.

5) (Also added later) Less than lethal weapon have less chance of causing serious damage to bystanders or those using the weapons.