THE FICTITIOUS AND HYPOTHETICAL SCENARIOS depicted by Marsden are largely unrealistic.
I recommend the hiring of retired trained military personnel who after extensive physical and mental evaluation are placed, armed, within schools (there are 135,000 in the US). Metal detectors should also be installed along with random searches of lockers, students, handbags and laptop bags.
Secondly... install two rugged keyed deadbolts on every classroom and administration door.
Lastly, psychologically evaluate everyone seeking to own a firearm. Those that are borderline, dysfunctional or crazed will have to acquire firearms illegally Possession of firearms by those failing psychological tests... 5 year mandatory prison sentence.