The fast-moving blaze at a Parkland School district garage in South Whitehall, near Allentown, was reported around 3 a.m.
People who live near the site reported hearing explosions around that time, and the cause of the fire remains under investigation.
At least 25 buses were destroyed in the blaze, which is roughly 25 percent of the district's fleet. No injuries have been reported.
As firefighters worked to bring the fire under control, Parkland School District Superintendent Rich Sniscak sent email and text alerts to parents and students saying classes were canceled. He said he hopes to have school reopen on Monday, and a few other local districts are offering to help get Parkland students to school with their bus fleets.
North Whitehall resident Chrissi Kent, who lives near the garage, told The Morning Call she heard "popping" from the scene for at least 45 minutes.
"Around 3 a.m. or a few minutes after, I heard a loud bang," Kent said. "Then it was quiet for a few minutes. Then I heard another bang. Shortly afterwards, I heard sirens. Then repeatedly bangs or popping sounds.