Cumberland County fire officials determined the blaze that broke out around 3 a.m. Wednesday was suspicious.
Investigators say there is evidence of a break-in in the back of the school, where a glass door had been busted out.
They are going over surveillance videos that captured several people inside the school at the time of the crime.
Hope Mills Mayor Eddy Dees says it shows more than one person, wearing hoods just before the fire.
Damage is estimated to be around $75,000 and the library will be closed for several weeks. Dozens of custodians worked to clean up the heavily damaged library all day Wednesday.
Investigators say gasoline was used to accelerate the blaze -- everything from books to furniture to computers was destroyed.
At some point, the culprits also tried setting a portable classroom on fire.
This is the third arson aimed at a high school in Cumberland County in the past five years.
Classes resumed Thursday.