I worked 20 years in public school in the U.S. and am quite familiar with emergency drills of all types, including intruder in the building, lockdown, call it what you will. I'm also quite familiar with crisis response theory/tactics. Lastly, quite familiar with the entire workings of mental health services, psychiatrists, psychologists, therapists, student assistance programs, children and youth services, juvenile probation, etc. etc.
In most of these shooting cases, I'll opine, as in this one, the shooter has a history. One of the problems is, and I repeat...ONE of various problems, believe it or not, money. When a child needs services, will the school pay? Or will Children and Youth services take the helm? Or will it be Juvenile Probation? When a child needs services, in particular, placement in another facility for behavioral or mental health concerns, somebody has to pay for that. The issue becomes quite complex in terms of need vs resources, parental involvement/approval, child involvement/approval, legal ramifications, service availability, etc. etc. etc.
Another issue becomes law relative to education. Public schools for the most part are charged with educating all school age eligible children who reside in their respective district, no matter the circumstance, be it language barrier, mental health, physical limitations or whatsoever. So, no matter the history of a child, that child, by law, must be in school.
In addition, law prevents exposure to, or the revealing of, a child's record, including mental health history, be it whatsoever, and normally those charged with providing such services, including counselors, therapists, school officials, etc. would never reveal details of a child's behavioral/mental health history to the public. (Some of the items found in these student files is quite revealing, being recorded and contained in each student's file).
Anyway, I could write volumes on the matter, but suffice it to say, each time I hear of any violent school incident, I pretty much know that there is a history with the shooter, and the details of that history are extant, but kept under lock and key, for various legal reasons.
One more item. I've always said, if you see it in real life, if it exists in our society, you'll see it in our schools. So, whatever you can imagine does and/or will happen on the streets of the U.S., chances are real good those things have, and will happen from time to time in public school....after all, the population in the school is but a sample of the general population, albeit young.