Dodge City! Probably already happened...

"The idea of 500 students in a college library or a dozen teachers in an elementary school pulling out guns to shoot a gunman is ludicrous. They would wind up shooting each other. "   From something Mike Williamson fisked. 

Well, one day that may happen. But it hasn't happened yet and been reported. I'm sure if it had Bloomberg's people would have publicized it so I can safely say it hasn't.

And it's not like 500 armed library patron would wheel at once on a bad guy and create a Mexican standoff.  It'd be onesy twosy.  Then done.