I take umbrage at the term "Active Shooter". I go to the range regularly, practice with rifles, handguns, and shotguns, reload, and attend firearms-related training at least annually. I'm also an NRA instructor, and teach firearms safety and proper use regularly. By all accounts, I'm an active shooter.
What the media calls an "Active Shooter" is really an active murderer (something that I am not).
With regard to the OP's scenario (engaging an active murderer with innocents down range) I am in the group that prefers to escape with my loved ones if this can be done safely. I'll engage, though, if I can't get away and the murderer brings the fight to me.
I suggest practicing headshots at varying distances from the kneeling position, using your carry piece, both with and without cover/concealment. The resulting angle reduces risk to downrange innocents. Try this exercise with your favorite mouse gun and also with a larger (but still concealable) pistol in a service caliber. For fun, try using a carbine or sbr. Consider which gun you'd prefer to be carrying in that scenario.
Going to the prone or "reverse prone" position as suggested above probably isn't a good idea; it limits your mobility too much.