In their defense, a lot has changed since then. Before Columbine, SOP was to form a perimeter and wait for swat. After the tragedy, SOP was changed to waiting until four officers could go in. Now it has changed to the first responder going in.
I can't remember where I read this, sorry.
Damn, I certainly wouldn't want to be going down hallways alone against an active shooter, especially if I don't know exactly where he/they are. Not very much cover in most hallways.