DeKalb County Police Chief Cedric Alexander said the incident happened inside the Ronald E. McNair Discovery Learning Academy in Decatur on Tuesday.
He added that the suspect entered the school by following behind someone who was authorized to be there.
“When we received the call and we engaged, he started to fire from inside the school at our officers. He had one or two of the staff members inside the main office. He was holding them captive,” Alexander said.
Police arrested a 19-year-old white male dressed in black and armed with an AK-47. His identity has not yet been released.
Alexander said that officers feared the man had explosives in the car or the building.
No one was injured and children are being reunited with their parents at a nearby store, officials said.
We're in the process of reuniting the children with their parents," said Mekka Parish, spokeswoman for the DeKalb County Police Department. "We're just trying to calm the nerves of parents."
The elementary school has about 870 children enrolled and houses students ranging from pre-kindergarten to fifth grade.
The shooting is the latest in a decade-long string of horrific incidents at US schools.
The deadliest came last December at Sandy Hook Elementary School in Newtown, Connecticut when a 20-year-old gunman - armed with a rifle and two handguns - shot 20 children and six teachers before killing himself.