Officials identified the alleged shooter as 20-year-old Michael Brandon Hill from the Atlanta area. According to ABC News sources, Hill was allegedly carrying an AK-47 knock-off and he was recently arrested for alleged "terroristic threats and acts."
All 600 students were accounted for after being evacuated from Ronald E. McNair Discovery Learning Academy, according to school spokesman Quinn Hudson.
Michael Thurmond, superintendent of county schools, said he was not aware of any injuries. The students were evacuated to a Wal-Mart, where they waited for parents or buses.
Two members of the U.S. Marshals Service were on assignment a few blocks away when they responded to the school. The two federal officers entered the school with at least one member of the DeKalb County Sheriff's Department. They said the suspect was armed with an assault-style rifle. They arrested the suspect without incident.
Assignment editor Lacey Lecroy at ABC affiliate station WSB-TV said she spoke with a woman identified as Antoinette, who works in the school's front office. She said the gunman told her to call and tell the media he wanted journalists to film him killing police. She told the station she was alone with the suspect, and the gun was visible.
"It didn't take long to know that this woman was serious," Lecroy said. "Shots were one of the last things I heard. I was so worried for her."
School officials could not confirm how many rounds were fired, or if the shooting took place inside or outside the school. A DeKalb County spokesperson confirmed the suspect fired shots, but did not have more details.
Authorities say Hill faces numerous charges including aggravated assault on a police officer, terroristic threats and possession of a firearm by a convicted felon.
ABC News and the Associated Press contributed to this report.