WASHINGTON—The nation's biggest gun-rights lobby called Friday for placing an armed security guard at every school, as it for the first time entered the re-energized public debate over gun laws in the aftermath of last week's school shooting in Connecticut.

Wayne LaPierre, executive vice president of the National Rifle Association, said that "the monsters and the predators of the world" have exploited the fact that most schools are gun-free zones. Other important institutions—such as banks, courts and power plants—are protected...