The fire was reported at 6:43 a.m. at the school, located at 25 El Camino Moraga.
When fire crews arrived, they saw smoke coming from a portable building that contains one classroom, Mentink said.
Staff working at the school helped firefighters get inside the classroom, where they found a fire in the attic space above the ceiling.
Firefighters were able to control the fire in about 15 minutes, and damage was limited to the classroom where the blaze originated.
The fire appears to have been caused by an electrical problem in the heating and air conditioning system, Battalion Chief Jeff Maxwell said.
Nobody was injured in the fire. The school is not in session this week.
Damage is estimated to be about $20,000.
