Some Northwoods Academy students arrived to find fire trucks at the Pierce Avenue school Thursday morning.
A school janitor noticed a smoky odor at about 7:30 a.m. and found a little smoke in a classroom, which prompted a teacher to pull the fire alarm, said Bibb County school system spokeswoman Stephanie Hartley.
Macon-Bibb County firefighters were called shortly before 8 a.m., but did not find any evidence of a blaze.
Crews inspected the inside of the school and isolated the problem to an air makeup unit on the roof that had a strong smell.
Hartley said a rubber belt got in a bind in the unit that draws in outside air, dries it and conditions it to be circulated in the building.
Firefighters did not find any smoke or fire, but the board of education facilities crew cut power to the unit until it could be thoroughly checked.
A couple of buses were diverted to Riverside United Methodist Church until students, teachers and staff were allowed into the building shortly after 8:20 a.m.