On body carry is the safest and most expedient method.
However it is done, having the weapon available when needed is a good idea. Personally, I would opt for a gun safe wired into an alarm system much like fire extinguishers are currently handled. This would prevent the inevitable instance of a weapon being left unattended or misused. It would also get the police officials involved earlier as well as alerting others in the building on on the grounds. I would also suggest looking at using rifles in lieu of handguns for accuracy and range. Something in the 30 caliber range would be ideal for defensive use in a populated environment.