A man with a gun enters a darkened classroom and begins shooting with mechanical precision: “Pop! Pop! Pop!”

None of the students move or resist, and within seconds the gunman has pulled the trigger more than 30 times. It is a virtual blood bath — and fortunately, just a demonstration — but it spotlights what could happen if an assailant armed with semiautomatic firearms and a will to kill gains entry into a school full of unsuspecting students and staff.