The fires were reported at 9:36 p.m. at Capuchino High School, at 1501 Magnolia Ave., and were extinguished sometime before 10:30 p.m., the fire department said.
Battalion Chief Joe Telles said the fires started in an area that is undergoing construction and has been fenced off. Arson investigators were called to the scene, and the San Bruno Police Department is assisting with the investigation because there was also evidence of vandalism throughout the school, Telles said.
The fires damaged two old cafeteria refrigeration rooms that the school had planned to tear down, according to Telles.
He estimated the fires still caused about $25,000 worth of damage because the school's water system was damaged during the firefighting operation.
Firefighters from South san Francisco, Colma and Millbrae assisted with the effort, Telles said.