Let me start off my reply by saying I am firmly on the pro- gun side of the gun control issue.
I am wary of arming any significant number of people in crowded, public places, especially closed- in places, simply because the potential for collateral shooting victims would increase, at least IMO. No amount of training or skill will prevent the shooting of the wrong people by mistake, or even the shooting of unintended people in addition to shooting the 'right' person(s) due to over penetration.
I hear what you are saying, and I do believe it would be effective but I am wary of the peripheral costs of such an approach, again in a densely populated, confined area.
The controlled entrance(s) and metal detectors method is also certainly not desirable in the least but I think it might be the only thing that would prove effective immediately.
The desirable thing would be to simply have people behave better but it does not look like that is possible. Which leaves any and all other methods as being undesirable but perhaps the only way to increase safety. Exactly the way air travel has very negative and undesirable security aspects now in an effort to increase air travel safety.
Frankly, I do not think there really is a 'good' solution to this problem and that is the most unfortunate aspect of all IMO. I do not like problems without solutions and suspect no one does but that is what this appears to be: a problem without a solution, only undesirable ways to address it with unknown costs, though they will be substantial, and unknown benefits.
Brian
Maybe we could start looking into having the school safety officers being armed.
Good guy with a gun stopping a bad guy with a gun.
Maybe even start letting qualified teachers conceal carry?
More good guys with guns to stop bad guys with guns.
I'm very sorry for all the parents and relatives who will have to live through this tragedy.
And, yes, I'm sorry if I have offended any gun control fanatics but my answer is in fact having more guns in the hands of good guys who have been vetted and are qualified to use them.