The following is a viral video regarding workplace violence put out by the City of Houston mayor's office.
"RUN. HIDE. FIGHT.® Surviving an Active Shooter Event"
https://www.youtube.com/watch?v=5VcSwejU2D0One of the items pictured is folks sliding something in front of the door to inhibit the entry of a shooter. However I wonder if sliding an object that wouldn't be too difficult to force out of the way (like putting the copier or little table in front of the door as pictured), wouldn't confirm to the shooter that someone is inside the room, whereas otherwise he might have glanced in hurriedly and not immediately seeing anybody have moved on. While expensive, it may make a not bad case for steel doors with locks on all the offices, to slow a shooter down (but also confirming the presence of someone in the room).
