The blaze broke out at about 3:45 a.m. in the school's administration building. Crews were able to contain the flames to that building.
It took firefighters about 40 minutes to knock the fire out.
Investigators said there were no electrical problems and they're convinced the fire was intentionally set.
Everything inside the building was a total loss. Damage is estimated at $250,000.
Authorities are now looking for suspects.
Anyone with information is urged to call the San Bernardino County Fire Department.