The fire broke out just before 1:30 p.m. Wednesday at the building at 7th Street and Lehigh Avenue.
Heavy smoke was seen on arrival and a second alarm was struck about 10 minutes later, followed by a third alarm. A fourth alarm was struck around 2:30 p.m.
The fire was declared under control around 3:30 p.m.
During the blaze, the smoke could be seen for miles.
"It just snowballed into a whirlwind of flames that covered the entire roof within minutes," said Taisha Figueroa of North Philadelphia.
The building was most recently known as the Julia DeBurgos Middle Magnet School but had been abandoned for two years with the electricity turned off and the windows boarded up.
Many people in the surrounding neighborhood went to the castle-like structure when it was Edison High School and they were sad to see it go.
"This is a piece of history here," said Dean Ortiz of North Philadelphia. "You got gargoyles in the front, you see the statue up top, looks like a castle. This community - we're losing that."
Firefighters say they found evidence that people have been inside the building. Now they want to know if this fire was an act of arson.
"That would be our first suspicion," said Fire Commissioner Lloyd Ayers. "But I won't make a call until my fire marshal tells me everything."
Some homes were evacuated for a short time due to the hot embers floating down from the fire.
There were no reports of injuries.