Authorities initially considered the incident at Vineland High School suspicious, but police said Friday that vapors from diesel fuel used to start the blaze apparently built up in a pit beneath a pile of wood pallets.
The explosion - which was felt or heard as far as 7 miles away - sent flaming debris flying through the air.
The only reported injury was to a Vineland firefighter who was struck in the back of the leg by flying debris. He was treated at the scene.
The pep rally for Vineland's Thanksgiving football game against traditional rival Millville continued at another location.