I believe it would be a mistake to hire armed guards for every school in the United States, in that one or two armed guards would be essentially ineffectual, yet would cost many hundreds of millions annually on a national basis. Further, the thought of single point entry is ludicrous in itself. This is a free nation. Remember.
However, were you to allow teachers, who were to go through the required training and mental screening (At this point you would probably also have many volunteer training programs!), to concealed carry, or even lock box access, to volunteer their services as first responders (not require it, mind you) ... you would see a dramatic increase in the amount of secure schools around the nation.
If only 5 percent of our nation's teachers were to volunteer in this respect (and I think the number of volunteers would be far greater than 5 percent) you would have enough security to forever deter cowardly acts of terrorism like these.
I would venture further to say this would work nearly anywhere security is an issue, without declaring and funding yet another war, this time on terror.