Authorities have identified the Jasper County High sophomore as Amanda Gaylynne Tanquary. Her body was found shortly after classes started. She appears to have been shot once in the head.
Authorities say all signs indicate that the wound was self-inflicted.
"We've had a self-inflicted gunshot wound at the high school at Monticello," said Ed Westbrook, emergency management director for Jasper County.
Georgia Bureau of Investigation spokesman John Bankhead said the girl was found in a truck, reports CBS Affiliate WMAZ-TV in Macon.
The truck was in the school parking lot, said Caroline Clauson, receptionist at the high school.
There were no apparent witnesses.
Westbrook said no one else was injured in the shooting. He refused to give further details.
The school is about 50 miles southeast of Atlanta. It has 536 high school students and 448 middle school students.
"We got the call after school started, so we're assuming it was after school started," said Judy Holloway, administrative assistant to the Jasper County sheriff.
In the wake of the Littleton, Colo., school shooting earlier this year, Georgia emergency management officials had set up a centralized unification site for just such an emergency, reports WMAZ.
Counselors are being brought in from the Oconee Child and Adolescence Center in Milledgeville. School officials say classes are continuing, but parents have been told they can sign their children out if they wish.
Busses are ferrying those students from the school to Monticello Baptist Church, where parents have been told they can pick up their children.