The incident happened at 8:30 a.m. Thursday at Phoenixville Area High School on Gay Street in Phoenixville.
According to Phoenixville Area School District Superintendent Dr. Alan Fegley, the student put his backpack on the floor of the classroom he was in and stated there was a bomb in the backpack.
"He was overheard making this statement by the classroom teacher," Fegley said in a written statement. "Emergency procedures were initiated, the police were notified and all high school students, faculty, and staff were safely evacuated from the building to Washington Field."
Video from Chopper 6 HD showed hundreds of students, teachers and staff sitting in the bleachers facing the school's football field.
Phoenixville police arrived and took the student into custody.
They then searched the school with bomb-sniffing dogs brought in from the West Caln Police Department and the Chester County Sheriff's Department.
The search turned up no evidence of explosives in the student's backpack or in any lockers, classrooms or other parts of the building.
Police gave the all-clear, students, faculty and staff were allowed back in at 10:40 a.m. and classes resumed. No one was hurt.
After questioning the student, police charged him as a juvenile with making terroristic threats and "other related criminal charges."