They watched him enter the school after firing rounds outside. They were on scene with a officer aiming a rifle at him prior to his gaining entry. 
This says a lot. They responded on scene and that officer acquired his rifle and took a position. 
This means many minutes passed while he was still OUTSIDE after police responded and they failed to stop what is basically an untrained child from entering a school full of children. 
The entire group of cops should be fired. Pathetic.