The average school shooter is not a current student at the school where the shooting takes place.
School shootings have nothing to do with behaviour in schools. Behaviour in school encompasses respect for self and others. There are schools in typically gun-supporting regions where behaviour in schools is perfectly appropriate and where school shootings have never happened.
I live in the midwest. My kids went to school in RC and public schools there. Up to the Columbine incident, they did not do shooter drills. They did both fire drills and tornado drills, though, from kindergarten on, and fire and tornado drills continue - do these count as potentially traumatic experiences? I know one of my DCs in particular had a lot of anxiety about tornadoes, but even though we've never had a tornado in this particular area and the schools they went to have never suffered a fire, I think it's wise that everyone in a school knows what to do in an emergency, regardless of how worried a fire or tornado drill makes any individual child.
They also test the public tornado alarms on the first Tuesday of every month at 10am - the alarm sounds like an air raid warning, the kind you hear in movies about WW2. Freaky.
School shootings do not 'affect' everyday life, whatever 'affect' means, in your post. People get up in the morning and get on with their days.
During the Cold War, kids did nuclear war drills and got on with their lives. I'm old enough to remember Mutual Assured Destruction.
Kids in Britain get on with their lives too, in schools where shooter drills are done.
People in Northern Ireland got on with their lives for the thirty years of the Troubles.
Elsewhere in the US, there are people living in regions where there are volcanoes, earthquakes, devastating hurricanes, regular tornadoes, floods. And there is traffic everywhere.
People can quake in their shoes and get paralysed by fear about every single thing that could go wrong in the course of their days. Most people don't. They get on with their lives.