I do not know what to tell you, but this security practice(s) in many schools is not really new, actually since Columbine, many of the states have had this practice.
What frightened me about the doors locking, not allowing outsiders in (as a mom and part of faculty staff) was now the idea of fire/disaster safety.
It's so sad that due to these horrible incidents, the lock downs, and daily locking doors, where say possible rescue workers would be delayed to assist those in need.
I wish I had an answer or reply to your question, but all that I was compelled to do with it was to share or tell others the fears that do not only involve things like what occured this week in Pa, and other states over the years, but the fears of safety against safety.
I shake my head and ask, "what's next? or what are we to do?"
Many schools in the US already have metal detectors at the entrances/exits, this is very sad, I come from an era that having chewing gum in your mouth or being caught smoking was the worst that happened . . . where has the future taken us?