Flames poured out of the building on the 100 block of Main Street in Nazareth at 1 a.m. on Tuesday.
Authorities say the fire burned through a Chinese restaurant on the first floor before it spread to the Lehigh Learning Academy next door.
No one was injured.
Officials say the school is closed on Tuesday, and six people who lived in apartments on the second floor are receiving assistance from the Red Cross.
The fire marshal and ATF are investigating.