A fire started shortly before 9 p.m. Friday in a classroom at Flory School in the mid-city area, the Ventura County Fire Department reported.
The first-floor elementary school classroom was fully engulfed in flames when firefighters arrived.
The fire was contained by 9:30 p.m., but another classroom also was damaged, fire officials said. More than 50 firefighters were on the scene late Friday.
The classroom is next to the offices of the Moorpark Unified School District at 30 Flory Ave.
Nearby is a storefront for the Ventura County Sheriff's Department.
The area is primarily residential.
Fire officials reported no injuries and said no other structures were immediately threatened.
A man who was not identified reported to deputies that he saw two people walking away from the campus area about the time the fire started.
No arrests had been made as of late Friday, but the fire was being investigated as arson, Sgt. Rick Barber said.
"Most of the time, not all the time, school fires are arson fires," Barber said. "They just don't start by themselves."
Four sheriff's deputies had been sent to the scene to investigate.
The Moorpark incident is the second fire reported at a school campus this week.
A malfunctioning heater in the administration building of Anacapa Middle School in Ventura sparked a blaze that caused more than $300,000 in damage Wednesday night.
That blaze will force the school's administration employees to work in another building for the remainder of the year.
Anacapa Middle School had been scheduled for a $3.8-million renovation project, so reconstruction due to fire damage will simply be added to the work, an administrator said.