Times and tactics have changed since then and we are now in the era of single-officer response, rapid task force response, and advanced trauma care training for patrol. Unfortunately we have seen incidents where officers have either not gone in, or have been ordered to not go in to handle the threat and save lives.
Nearly two decades after Columbine there shouldn't be any question as to what we as law enforcement should do in this situation. We have to respond as safely as possible, grab what we have with us, and stop the killing.
Interior Vs. Exterior
The tactics I recommend for response are the same ones I teach for the National Tactical Officers Association (NTOA) through the Advanced Active Shooter Response Instructor course. We include classroom presentations, MACTAC (Multi-Assault Counter-Terrorism Action Capabilities), exterior response, interior response, and instructor development.
Your active threat program should include response to exterior threats such as what we saw in Boston following the marathon bombing. What if your threat goes mobile, into the residential subdivisions of your jurisdiction? If that happens your interior tactics are out the window and now you need a new plan of action to resolve this issue. Exterior tactics such as bounding, staggered columns, wedges, peels, flanking, and directed fire should all be included in your active threat response program.