So the siren tells you to do what? Go back to your dorms (where the guy had already killed people?) Go to your first class and stay in your room with the doors locked (I don;’t know if the classes had locks, but if they did the guy must have been able to shoot them open).
And when do you stop the siren? 2, 4 hours? All day? Until they solve the shooting and arrest the gunman — some murders take weeks or months to solve?
People already are saying that it is very ODD for a shooter to wait over 2 hours before shooting again. So even if your initial inclination was to lock down the whole school to make sure the gunman wasn’t running from dorm to dorm, by 2 hours you probably would have eased up on the lockdown if you hadn’t had any contact with a gunman.
And in fact, they DID lock down the nearby dorms, AND alerted the other dorms. And they thought they knew who the person was, and where he was driving.
False alerts do have a cost. The school was locked down 2-3 times the previous week, and the kids were paying good money to be taught, not to be locked in their dorm rooms.
Think of this. If there had never been a 2nd shooting, and if instead the shooter had been caught hiding in a building 4 hours later, would everybody be screaming that the school should have been locked down?
Given the escalation of violence - from mere bomb threats to actual corpses - the security apparatus should have realized that there may be a larger problem at hand. They can't just play the "boy who cried wolf" and decide, "nah, not this time. We've got paying students to educate." The results are painfully obvious.