The victim, 14, was talking to investigators at Grady Memorial Hospital, where he was being treated for the wound, sustained just before 2 p.m. at Price Middle School, The Atlanta Journal-Constitution reported.
Police said he was "alert, conscious and breathing," the newspaper said.
The victim's mother told CNN her son knows his assailant. She said the other student had been "talking smack" to her son before pulling a gun and shooting him.
Police said they recovered a gun at the scene.
One teacher sustained what police called a minor injury while running, but no other students were injured.
"Gun violence in and around our schools is simply unconscionable and must end," Atlanta Mayor Kasim Reed said in a statement. "Too many young people are being harmed, and too many families are suffering from unimaginable and unnecessary grief."