Massive drama but low probability; twice as many people are killed by lightning strikes every year. Beyond that, the "mass shooting" tallies often include gang fights and cartel wars, which most of us do not need to worry about.
Low probability, but it happens each year and several times. Just in 2021 it happened 32 times. I bet that each time a mass shooting happens in a school it affects emotionally to all teachers in the country.
As teacher is normal to deal with problematic, or even very twisted children sometimes, but the feeling in Europe is very different. I can perfectly understand somebody that leaves after experiencing such traumatic events non-stop each ten days.
Because it’s something you can actually react to. There’s almost no chance a tornado will hit your school but schools do tornado drills. All schools and businesses do fire drills. There was a time they did nuclear bomb drills and many buildings from the era have fallout shelters.
So yes it’s unlikely to occur. But it could and it requires very little effort to be prepared.
The main effect of these drills is to cause kids stress and anxiety, but I also wonder if it's possible that there may be some marginal benefit of increasing awareness and vigilance of warning signs among the kids and teachers.
Older American folks here may remember doing nuclear attack drills in school, where the kids were told to crouch under their desk.