Indeed, having armed security in our schools is a local issue. Some school districts will choose this option, while many will not choose to be armed.
Having an armed presence is a potent deterrent. As mentioned in another thread, since the Israelis implemented a policy of having armed security at their schools in the 1970's, they have experienced zeo mass shootings at any school.
To the best of my knowledge, all mass shootings at American K-12 schools have taken place where no armed security was present.
These shooters want soft targets where they can control the outcome until their bloodlust is satisfied. Cowards, one and all, they kill themselves as police close in. These are suicide missions from the outset. In the case of Adam Lanza, his to-do list was matricide-homicide-suicide.