"It's a blessed day: All of our children are safe," said Michael L. Thurmond, interim superintendent of the Dekalb County School District.
Police continued to investigate the car the gunman drove to the school after bomb-sniffing dogs detected explosives. Police later blew up the car's trunk and towed away the car, but found no explosives, a police official said.
Authorities believe the man may have entered the school by closely following a person authorized to enter the building, however once inside, he only went as far as the front office.
The shooting came on just the second week of classes at the charter school.
School safety and gun control were again thrust into the national spotlight last year when Adam Lanza, 20, killed 20 students and six adult staff members at Sandy Hook Elementary School in Newtown, Conn.
ABC News' Diane Sawyer, Mike Levine, Steve Osunsami and Michael S. James contributed to this report.