"Sandy Hook's doors were reportedly secured, but the shooter still managed to somehow gain access. "
Steel wrapped wood framed 45 minute fire rated security doors perhaps? AR-15= skeleton key. 20 minute fire rated standard steel wrapped door? 9 mm pistol = skeleton key. Non steel wrapped hollow core door? steel towed boot = skeleton key.
Reinforced steel frame double-bolted 1 hr fire rated security door (2-300 pounds each for ADA requirements for wheelchair accommodating width) = prison camp with kids and perhaps small teachers unable to open the doors safely and easily under normal day to day conditions.
And then there are windows and for a wood framed building even exterior walls that can be breached on either side of the prison door