Woodstock High School and Woodstock Middle School were locked down after the gun was discovered, WGCL-TV in Atlanta reported. Deputies from the Cherokee County Sheriff's Office searched a home and found two assault rifles and ammunition.
Forrest Busby, 17, of Canton, who allegedly planted the gun, was charged with carrying a concealed weapon, possession of a gun on school property and other crimes. Two other teenagers were also arrested.
Katie Mitchell, a senior at the high school, said she arrived at school only to be told to hide behind a trailer.
"And then a few minutes later, a teacher ran frantically and told us to run to the gym," Mitchell said. "It's really scary. School's supposed to be safe, so to find out something like this is happening is really creepy."