They say lock doors, barricade, and hide.
I don't know how effective that is.
I realize this isn't the "sexy" solution of fighting back H2H or with your CCW, but it is the most effective strategy on an individual level. Think about it from the perspective of the active shooter: What is your goal? How long will you try to break down a locked door? (sub-question-how long will you try if you know someone is in there vs. how long if you have no idea?), Also, consider the construction of commercial grade doors in buildings. Consider police response time and that with every second spent looking or beating on a door, your other potential targets are getting away and the police are en-route...
Hiding behind a locked door is the best individual strategy if escape cannot be guaranteed/feasible. The best strategy for group benefit is taking them out...at great risk to the individual who tries it, armed or not, especially if "not."