Atlanta Police Officer John Chafee said Marcellus Brooks was being held at the Fulton County jail after the shootings.
Brooks is accused of opening fire about 100 yards away from the school at about 4 p.m.
Atlanta Public Schools confirmed that four victims were Therrell students. None of the injuries were believed to be life-threatening.
Three of the injured were still hospitalized late Tuesday. One, a high school senior, had a bullet lodged in her leg, the girl's mother said.
The school was on lockdown Tuesday in response to the shootings, and Atlanta Public Schools announced book bags and backpacks would be banned for the rest of the school year out of precaution.