I'm not sure where you would really go with this, to be honest. I mean it's not a setting that's going to last for any real length of time, the shooting would last for like tops a half hour or so until the shooter was either stopped or had killed everyone.

I dunno, it feels to me like if you wanted to explore the niche of "regular people thrust into a dangerous situation", then you'd be better off with some sort of apocalypse RP. It would have the same impact on the characters, be less offensive to people, and be a setting that won't have a very limited time frame.