One effective solution could be trained people being employed as armed security.
If they have clean records, military experience, past police experience etc. then that's a possible solution.
Things have greatly changed within today's high school in the US.
Today a fair number of High School's service a large area and have thousands of students. It's not only a high school during school hours............ it's a city with three thousand students.
If you have a small community with 1,000 to 3,000 residents there will be some level of policing. It might be a sheriff's district or city police force or State Troopers.
A community with 3,000 residents will have there own on site police force.
Considering the frequency of attacks in schools........ I believe that at a certain level a High School should be patrolled by active trained on site police officers. Officers who actually share some part of the school.
At this point in time no student in the USA is protected during school hours.
Where are the metal detectors? Emergency alarms that call for an immediate police response....... not just the fire department.
Hallway cameras everywhere, reinforced classroom doors, speaker phones to report a problem and of course an active police presence so a response is there in minutes.
Historically we have done pretty well with fire proofing our schools but we have failed to gun proof them.
And one final issue......... In each and every gun assault on school children there was no immediate active medical treatment available. A child gets to lay there bleeding out. There has to be a trained emergency response on site........ not just the school nurse.