The blaze was reported around 9:30 a.m. Monday at Ben Franklin High School in the 500 block of North Broad Street.
The fire alarm went off, and the entire building was evacuated.
"The first company arrived with smoke showing from the 4th floor," said Philadelphia Fire Department Capt. William Dixon.
The blaze was quickly brought under control.
Investigators tell Action News the fire was contained to Room 402, one of the school's science labs, where the blaze started.
As the investigation got underway, school officials decided to shut down the school for the rest of the day.
Students, who were relocated to the nearby Franklin Learning Center, were dismissed. Some remained at the center, awaiting parental pickup.
School officials say, all things considered, the situation could have been a lot worse.
"No injuries, great news," said Philadelphia School District spokesman Fernando Gallard.
School will reopen as usual Tuesday.