Procedure since Columbine is the first officers on the scene must go in immediately to try and stop the killing, even if they are alone. The killing is usually over by the time more officers can arrive and get into the building. Hence what some call a ‘suicide entrance” has to be made.
“But what’s clear is that police and school security personnel now believe that while police Special Weapons and Tactics teams may be better equipped, waiting for them should not put students at risk.” you are correct, it’s preferable to have 2-3 entering, but if only one is there then only one is there.