There is a reason terrorists and people with Mental Illness pick gun free zones instead of Gun Shows and Police Stations to target.  They know that even if you have a guard, once they are taken out they can shoot fish in a barrel.  Navy Yard showed us that.  Law abiding citizens ARE the first responders.  Most of Europe is nearly a gun free zone...it didn't work out well for them during the Charley shooting.

This may sound good on paper, but will cost people their lives at some point.