Authorities say 42 students and six adults were taken to hospitals and about 500 students evacuated from Finch Elementary School.
Firefighters were initially told people were unconscious at the school, but Atlanta fire Capt. Marian McDaniel tells The Associated Press that all the patients were conscious and alert as they were being taken to hospitals.
News footage showed young children strapped to gurneys with oxygen masks on their faces and emergency officials carrying others in their arms.
McDaniel says firefighters detected high and unsafe levels of carbon monoxide near a furnace at the school. She called the reading of 1,700 parts per million extremely high.
One expert says the colorless, odorless gas can be deadly at that level.
Initial symptoms of carbon monoxide poisoning can be confused with the flu since both include malaise, headache, nausea and vomiting.