It happened around 12:40 Wednesday afternoon at the school on the 500 block of North Broad Street.
Crews arrived to find an active fire in the school's weight room located in the basement of the building.
The flames were brought under control within minutes.
No one was injured and students and staff were evacuated safely.
The School District of Philadelphia said via twitter that classes would dismissed early and that the school would reopen Thursday.
Police have blocked off the area around Broad and 15th streets in both directions from Spring Garden Street to Wallace Street due to fire activity.
No word yet on a cause for the blaze.