If other shootings are any guide, the police would have stayed outside the building until they thought they had better control of the situation. That is how so many people died at Virginia Tech, the police didn’t want to take a chance that they might get shot. Also Columbine.
Oddly enough, law enforcement is learning from all of these shooter situations. So far, most have offed themselves as soon as they were met with armed force.
The original tactics were to activate swat study the blueprints of the building, form a committee to discuss what to do and then go in and collect bodies.
The thinking is moving more to storm the doors as soon as the first officer arrives and engage the shooter.