You'll recall I said I'd think about writing a piece on engaging an active shooter in a sort of "worst case scenario". Well, the truth is that I can't teach you that over a blog. Hell, I might not be able to teach you that in person. Yes, I've taught urban warfare classes (on tactics, not strategy) in the Army at the platoon and company level. I've been in a gun fight before, and came out on top. I read everything I can get my hands on about tactics, techniques, and theory behind shooting. I intend to go to a private shooting school and take tactical pistol and carbine courses once I get a real job. I have experience, training, and an adequate level of marksmanship. I fancy myself a journeyman gunfighter--not a master of anything, but capable or even good at a few things.
Engaging in a gunfight is something that you must weigh against your training, experience, skill, mental agility, and personal moral code. I cannot advise you on how to proceed. There are just too many variables. Yes, this is ducking the question a bit, but a wise man knows when he is out of his league, and advising on this is definitely not something to do over the interwebs. In lieu of my advice, I will point out a list of reading materials, and some places you can go for training from an instructor--not just a gun nut from the intertubes.
SWAT Magazine: Great for gearheads who have to know about the latest and greatest tacticool toys. It also generally gives sound tactical advice and is written by dudes who have been there and done that.
Anything by Col. Jeff Cooper: Cooper lived a wild life, from the Marine Corps to intervening in conflicts in Africa on his own time, as well as being a truly great big game hunter. He also championed the 1911A1 pistol and pretty much wrote the book on self-defense. He was a great man and his books are very informative. He has some strong opinions, and occasionally he and I differ, but I'd never say he was wrong.
Lethal Force Institute
Of course there are many more, but these are probably the most popular/famous.
Learn, train, then practice.