My high school had an event. Students were stuck where they were for about 4 hours, although they forgot about the dance portable and they were trapped for 8 hours or so. Clearing an entire high school takes a lot of time.
School shootings don't happen as frequently as it seems like. They happen too often, but the media coverage makes it seem like it happens all of the time. The actual need for armed security is not really there, and the potential problems from that could be statistically worse than school shootings. I am not for gun control, but I also do not see any way to rationally argue that introducing guns into school environments is a good idea. Accidents would probably cause more deaths per year than school shootings.