They didn't know how to react at Columbine. Since then policies have been developed where police go right in to confront the shooter(s).
I think they are required to go in in groups of three and not as individuals, so there may be justification for a lone officer to wait. However the urgency is such that individuals from different agencies are to group together and go in if they are the first ones on the scene. There certainly was no justification for a group of officers to stand around doing nothing.