I agree with Uncle that, given the data we have now, the police acted appropriately in not locking down the school after the first shooting.  They had sufficient reason to believe that this was an ordinary murder, and that the killer was at that point on the lamb. I don’t think the police could have reasonable anticipated the killer would then start a mass killing spree.
The police are not supermen.  They are ordinary people who make ordinary mistakes, and have to act, quite often, on spotty information. Once those folks were chained into the room, there wasn’t much the police could have done. Even a SWAT team would have taken time to blow down the door. As I said before, the only person to blame in this tragedy is the deranged lunatic. It’s foolish to expect others to provide perfect safety.