Firefighters battled a blaze Monday morning at Benjamin Franklin High School on North Broad Street.
No injuries occurred in the fire, which was reported at about 9:50 a.m. It was declared under control at 10:14 a.m., the fire department said.
The school at 550 N. Broad St. was immediately evacuated after a teacher reported the fire in a fourth-floor classroom.
About 450 students were in the building at the time, Philadelphia School District spokesman Fernando Gallard said. Most of them were relocated to Franklin Learning Center, a few blocks away.
The fire was confined the classroom. Initial reports indicated it broke out in the ceiling area, officials said.
The cause of the blaze wasn't immediately known and the fire marshal's office was continuing to investigate how it started.
The students are expected to be dismissed from school early.
Gallard said the district would determine later Monday whether the building would be ready for classes on Tuesday. The extent of damage to the school wasn't immediately known.
Gallard praised the school staff for quick thinking and a smooth evacuation.
"A lot of credit goes to the teachers," he said.