The fire broke out around 2 a.m. Wednesday at Sacred Heart School on the 300 block of North 4th Street.
The 115-year-old school sustained major damage to the roof and portions of the 3rd and 4th floors officials said.
School principal James Krupka says it appears the worst of damage is in the attic.
"Obviously, the water damage cascading down the floors. There are computers in the computer room and teachers may have had supplies," Krupka said.
Sacred Heart is located directly next to Allentown Central Catholic High School. Officials said the high school may have suffered smoke damage.
A firefighter's legs went through part of a floor while fighting the fire, officials said. He was checked at the scene and is said to be OK.
No other injuries were reported.
"I'm just pleased and thank God they're all safe," Krupka said.