From the Atlanta paper:
All ammunition is banned from the building, said Warren, who declined to release the instructor's name or any initial findings as to why he had the pistol. Warren provided no further details of how or why the gun fired.
Further, a state policy bans any working firearm — loaded or not — from academy classrooms, said Bob Sanderson, assistant director of the Georgia Public Safety Training Center in Forsyth. The state oversees the center in Austell and nine other regional police academies.
"In the classroom, they use what is called a red gun, models that are made of red, hard plastic that are replicas of actual handguns," he said.
A lot of old schoolers think the rule is for someone else. Since the department isn't releasing info, you're left with speculation, but my GUESS is that he carried his own weapon into the classroom, and was probably the highest ranking officer there. You generally don't tell higher ranking officers that their breaking regs, and few civilians will tell an officer to remove his sidearm when he enters a building.