Metal detectors are not required in all schools. However, with that said, yes this one was preventable. He told people what he was going to do. The primary person he threatened stayed home because he feared for his safety. Unfortunately no one turned the kid in to authorities until it was too late.

That and he had an irresponsible dad who had just bought a gun and did not lock it up. So, yes, it was preventable.

There are lots of reasons it is becoming commonplace. However, you should note that it isn't that much more common than in the past, it is just much more published than before. One of the main reasons it is happening is we have failed to teach kids how to properly use and respect guns. We have failed to teach kids to respect others. And, we have taken guns out of the hands of teachers and other school personnel. Shoot, when we were in school everyone had at least one gun in the gun rack in the back of their unlocked pickup at school, but nobody would have dared use one because we all knew that most of the school staff were armed and knew how to shoot, and we knew that everyone else had a gun too.

With all that said you might be interested to note that school deaths are actually down from where they were in the early 90s.

https://www.cdc.gov/violenceprevention/youthviolence/schoolviolence/savd.html

There is a graph part way down the story that shows school deaths by year from 1992 to 2016. You will note that the two highs on the graph are in 1992 and 2006. The one big change is that a higher percentage of the violent deaths involve multiple deaths, but that there are fewer total instances of violent death. In fact, 2016, the last year on the report was well below the average deaths per year. So, much of what you are seeing is actually perception, and not necessarily fact.

Someone commented:
“Shoot, when we were in school everyone had at least one gun in the gun rack in the back of their unlocked pickup at school, but nobody would have dared use one because we all knew that most of the school staff were armed and knew how to shoot, and we knew that everyone else had a gun too.”

We didn’t have school shootings when I went to school either. No one had guns, students or teachers. From these conflicting positions it might appear that there are two ways to prevent school shootings: Everyone has a gun or no one has a gun. I prefer the latter.