As a parent myself, I can't think of a worse phone call to receive, other than "your child has been killed" is "your child murdered his classmates."
Many of these shooters grew up in dysfunctional homes but others didn't. Growing up in the shadows of Columbine, I often thought of the parents of the killers, seemingly well-adjusted, intact middle-class homes, what we call "normal" families. They had children to bury as well, children they loved and thought they raised right.
Sometimes it is very apparent a child has a mental illness and violent tendencies. I think parents of such children should at least inform local teachers and law enforcement so they can at least keep an eye on those children’s behavior. Not make a federal case about it, but just actively monitor if the child/teen suddenly starts to act really differently. It may be something the child does at school that the child hides from the parents.
For some of these child murderers though, there isn’t any real warning or the signs are hidden. I mean, if your teenager is keeping a diary of violent thoughts in his room, it is likely to be hidden in a place where parents would never know, especially if the kid only writes the diary in secret. A parent would never know of the existence of a diary. Ditto for collecting violent articles or producing/consuming violent art. A kid may hide all of that from parents. I don’t think most parents go around looking for evidence their kid is mentally ill with violent thoughts if there is zero evidence to even suggest it. The kid may be a model child while privately harboring these thoughts or notes or art.