Firefighters responded to the school located on East Chapman Avenue just after 2 a.m. following a report of a ringing alarm.
When units arrived on scene, they found a smashed glass door and heavy smoke surrounding a large portion of the school. Fans were brought in to air the hallways.
Officials said the fire was contained to an office desk. The blaze charred the desk itself and the ceiling above it. A trophy case was also smashed. Several other items were damaged but nothing was taken.
Fullerton Union High School Principal Rani Goyal called the incident a smash-and-go and a vicious act of vandalism.
Classes were expected to not be affected Tuesday as Goyal said the fire was restricted to a main office classroom. But crime tape surrounded the building.
No word on the possible suspects or the amount of damage caused by the fire has been released. Officials say the school does not have any surveillance cameras.
The fire itself is being called suspicious by law enforcement but that may change as the investigation continues.