What prevents them from doing it now? The thing that will stop a crazy with a gun, teacher, milk truck driver, kids, whatever, is another person WITH A GUN.
Thats the only thing that works! Who is in school all the time? Teachers/students
Nothing will stop a crazed dumb**** or anyone else from killing students/teachers, except the knowledge that Bob, John, Suzy or Mrs. Daisy from History across the hall MIGHT be armed, or better yet, they KNOW other students/teachers could be armed.
Why don't the fruity loops attack places where they know a gun could be? Because guns are a deterrent! Boo
It's why PDs are rarely attacked by crazy people. Even insane people get it.