We found that the more complex we got, the less likely it was to work. Don't worry about the other stuff. Focus on removing the unstable event (the shooter). Keep it as simple as you can. The major factors we found were:
1) The first officer on scene, regardless of rank, establishes Incident Command (same as any other Critical Incident) and sets up a staging area;
2) The IC forms an entry team out of the first responders. This should be as many officers as you can get, but the IC needs to balance waiting for more officers to show up against stopping the threat immediately. The average active shooter incident lasts only 20 minutes. This is not within the typical SWAT response time window. Patrol will have to stop the intial threat. Columbine, an anomaly, lasted 2 1/2 hours until both shooters were dead; a quicker response could have saved lives.
3) Forget a perimeter (for now). Forget EMS, or any evacuation. The threat needs to be stopped. Period.
4) The entry team makes entry and goes directly to the threat. They should not stop to help anyone or mess with IEDs, and your training needs to incorporate those aspects. The team organization and entry style is best left to your Department and should at least in some way mirror your Department's current entry styles, but again, simpler is better.
5) The team locates the threat, goes directly to it and stops it, period. This is the tough one. The first time the team takes serious fire, they will freeze up (I have seen it from both ends dozens of times). This is why training (especially using Sims, bangs or another training tools) is so critical. When the team freezes or stops (they usually pull into the nearest room or cover when they start taking fire) they get shot to pieces. They need to drill assaulting through the threat, laying down supressive fire and not stopping until they do it without hesitation.
6) Evacuation of wounded and the uninjured can be accomplished after the threat has been stopped or contained. If the threat is contained and doesn't pose an immediate risk, the situation slows down and gets handed over to SWAT as a "standard" barricade/hostage incident. If the shooter is down/in custody, the IC either forms secondary entry teams or hands evacuation off to SWAT. SWAT may need to send their tactical medics in or escort EMS for critical care. In any event, SWAT will have to clear the building again after the shooter is stopped. expect this to take several hours.
It is also critical that any other agencies in your area who could be reasonably expected to show up are trained in your methods as well.
Last time we did active shooter training (we do it every six months), I was a "bad guy." I got hit about 800 times (my best guess) by Sims that week. I had a fingernail shot off. Expect to use alot of Sims and bangs, and you will have to experiment to find a method that fits your agency, equipment, and likely scenarios.
Hope this helped somewhat.