The one thing I'd point out is that when you look at mass school shootings of the past, they are happening in rural and suburban, sometimes affluent areas of the nation; and I don't recall one of those schools having a building entry screening process in place.
Like everything else, it requires balancing security with the mission of the institution. Here in King County, Wash., there were calls for more security at the King County Courthouse. It took two deaths from one shooting incident to put the screening process in place. Now 20 years later that screening process remains in place.
I don't have easy answers.