Tucker Thayer was wounded in the head when the blank-firing gun went off, said St. George Police Sgt. James Van Fleet.
The 15-year-old student was a stagehand for the play at Desert Hills High School, his father Ron Thayer said.
Tucker was one of a few people preparing props in a sound booth before a planned performance of the play, Van Fleet said. No one saw the gun go off, but they heard the gun fire about 6:20 p.m. and called police.
It is unclear whether an adult was nearby at the time. The guns were supposed to arrive at the school under lock and key, and not be available to students until they were needed for the show, Van Fleet said. Police are investigating how the teen got the firearm.
"Why was a 15-year-old handling a gun with no supervision?" Van Fleet said. Police do not know whether the shooting was accidental or intentional, or if the boy was unaware blank cartridges' are a potential danger.
"We're not ruling out anything at this point," he said.
The teen was flown from Dixie Regional Medical Center, but died sometime before 10 p.m. His name has not been released.
"Our thoughts and prayers right now are with that family," said Craig Hammer, executive direcor of secondary schools for the Washington County School District. Grief counselors will be available for students Monday.
Last month, Desert Hills High School was locked down after Tucker was seen with another gun from the same play, his father said. Tucker had taken a rifle replica home to fix it, and as he returned with the gun, someone saw him and called police. The school was locked down for about an hour on Oct. 10, Van Fleet said.