James Alan Fox nails it. "Commercial airlines train their flight crews to handle disaster situations—such as the unlikely 'water landing'—but passengers are only asked to watch a brief demonstration of grabbing hold of oxygen masks, without having actually to practice this maneuver. Cruise ships require that guests don life jackets and learn the location of their muster stations, but no one has to step foot inside a lifeboat or suffer the experience of being lowered into the water. In case of a catastrophe in the air or at sea, the passengers will be directed where to go and advised what to do.
This same reasonable posture should apply for schools: prepare the staff but spare the students. As with the usual pre-flight or pre-cruise protocols, a few simple instructions on escape strategy may be sensible. However, over-preparing students needlessly risks intensifying their fears and anxiety."
I can only think of a few of reasons this would go on, i. ignorance, ii. state indoctrination and fear mongering and/or iii. both of the aforementioned.
(Via.) Hit & Run : Reason.com