Pine Bluff’s police chief, Kelvin Sergeant, said investigators think the sniper is a 15-year-old man who also studies at Watson Chapel Junior High.
The suspect was located in a neighborhood close to the school, hidden behind a house.
He was taken into custody and is currently being held at the juvenile justice center in Pine Bluff, officials said. Formal charges have not yet been made.
There is no known reason for the shooting, said the sergeant, who called it an isolated incident. The school followed the protocol of the active sniper, the sergeant said.
In a Facebook post, the school system said students are safe and schools are being closed.
Pine Bluff is about 72 kilometers south of Little Rock, the state capital.
Correction: This story has been updated with the correct age of the victim. The victim is 15 years old.