Yeah I'm more just wondering is it possible/how hard is it to blast through a deadbolt on a metal door/frame with a shotgun? FWIW I'm not trying to condone the actions (or lack thereof) of the officers, I'm just curious (should I ever be in the same situation).
Whether it's shotguns or any other breaching tool, they're supposed to go in and have the equipment they need to breach the door. It should be something they have with them the second they get in position.
As BigL stated, it seems clear they hadn't done any training at this school, or even any school. And you use what you have to to gain entry and neutralize the threat as quickly as possible.
And to be sure, there are several different types of shotgun shells, and there are some capable of creating enough damage to the door hinges to breach in very short order.