The inimitable Glenn Reynolds has a good essay on what is likely the only reasonable protection against random mass shootings on school campuses and other vulnerable public places.
Dave Kopel's Wall Street Journal op-ed says much the same, with somewhat more background and context, even.
It is almost certain, though, that the one safety measure that could work effectively will never be implemented. I do expect we will see a massive amount of discussion on things that won't work and that more impotent policy will probably be enacted. Sadly, there will probably be some good people scapegoated in Virginia before it's done, sent undeservedly into forced retirement or otherwise disgraced. Such gestures are merely symbolic, promoting an illusion of efficacy, but meaning nothing and doing nothing to make anybody any safer.