The Second Amendment to the Constitution was created to recognize the fact that the militia is "the people" as stated in the amendment. US law says male citizens between ages 17 and 45 and certain females (in our modern times it should include all adult citizens).
The Second Amendment states that the militia must be "well regulated" which meant "in proper working order". A militia that is in proper working order must be well armed and must be well trained. They will keep arms (at home) and bear arms (in public when needed).
A militia, as defined above, would have quickly stopped the tragedy in Newtown. It is time to recognize that all adult citizens have the responsibility to be part of a well regulated militia with extra training required to carry concealed weapons in schools.