Donald Trump wants to arm teachers with guns

[A Theist]

The armed officer at the school, Scott Peterson, resigned yesterday amid the revelation that he did not go into the building and engage the shooter. He stayed where it was safe for him while students were being killed. The shooter was well known to the local authorities and the police had been called to his home multiple times. Even the FBI was warned about him. There was a total irresponsible and ineptness of the local authorities and the FBI to act against the shooter before hand when they already had the information on him.

Given all this, it seems the only viable long term course of action in preventing another similar event is to... make sure that the gun never reaches the person, perhaps?
And, since you can't predict who will end up doing this again, maybe it's not a bad idea to minimize the number of people with access to guns.

How many illegal guns that the police and local authorities don't know about would you estimate are in the hands of criminals in our cities? How would the anti second amendment proponents go about getting those off the streets?