In the school district I work for, there have been a few active shooter drills, but everyone was informed ahead of time (including the police and the staff at other sites, because radio communications can be picked up between sites that are close together).
For that matter, all drills are planned ahead and the staff is informed that they will happen (excluding a few unplanned fire drills caused by students pulling the alarm). If an unexpected lockdown is announced, we know there's a reason for it. It may not be a shooter, but there's some sort of danger.
I have been present for a few incidents where there was some sort of danger (or the perception of danger). When I was a freshman in high school, shots were fired at the apartment complex across from the school and we were placed in lockdown. When I was a sophomore, some students wanted to start trouble because of the rioting going in L.A. (the Rodney King incident). When I was a senior at the same school, there were two small-scale riots on campus. At the first school I worked for (the same high school I graduated from), we had frequent lockdowns because morons would hold up the fast food restaurants and gas stations across the street from the school, then run on campus to try to hide. Someone also set off a homemade bomb one day (it didn't do any damage, but it did scare the heck out of people). At my current school, we had one attempted murder (a kid lay in wait for another kid and bashed him over the head with a pipe), one incident where several cops surrounded a nearby house, guns drawn (so we were in lockdown so no dumb kid would try to get into the middle of things), and one incident where someone apparently fired a gun into the air and the bullet came down and shattered one of the library skylights. In every case, some people got panicky (students and staff) and had to be kept calm and out of danger. Rumors flew in every case, too, blowing things out of proportion (and they flew just as fast pre-cell phones as now).
Because of all this, I can definitely say that having an active shooter drill without informing the staff and students first is a very bad idea. People will panic, some people may be traumatized, and some people may try to defend themselves and others, which can end very badly.
As far as a fanfic challenge goes, I can definitely see this used for a nextgen fic, or even a fic about Lois and Clark (as teens or as adults; I can easily picture Superman hearing the screams for help and rushing into the middle of a drill that people weren't informed about).
As an aside, how could the school authorities be so dumb, especially in the wake of Sandy Hook? Situations like that are really scary. Heck, even in theater, where everyone knows the gunshots and the threats aren't real, the are very strict rules about who may handle a prop gun so there are no mistakes and no accidents.
