It’s not urban warfare just because it happens in a building. Is an armed robbery of a 7/11 urban warfare? Is domestic violence in a single family home urban warfare? The answer is no in both cases, and also no in the case of a school shooting.
You might be able to argue there are some similarities to some aspects of urban warfare, but they are not the same thing. Trying to equate them, so that you can say the victims of a school shooting should have behaved in a way different from how they did, is just reaching.
I've said nothing regarding how the victims should react; I've said that the victims would react the same way in urban warfare, as well in no-knock raids, as well as a school shooting like this. Precisely because all of these are employing a rapid-dominance strategy (on the part of the soldier, the police, and the shooter; respectively).
Is that reaching?