there was an armed guard at Columbine,but he was out to lunch at the time,and the killers KNEW that. Guards are known and easily planned for.
Having armed teachers (and other school employees)is like having plainclothes police scattered all around the school,at no cost. it's the smart thing to do.
it's also a DETERRENT,because these mass murderers SEEK OUT "gun-free zones" such as schools. If it's known that schools have armed people,the murderers will avoid them,as the Aurora murderer did;he bypassed theaters closer to him for the one that was "gun-free".