The one-alarm blaze, reported shortly before 6 a.m. at the school at 3115 Del Monte St., started outside the metal locker and spread inside, prompting investigators to label it suspicious, according to San Mateo Fire Department Battalion Chief Mike Borean.
"It started on the outside, went underneath ... and worked its way up to the wood floor," Borean said. He also said there was no heat source there to trigger the blaze.
The fire never burned out of control.
"We stopped the flames almost immediately," Borean said. "It was pretty mild as far as what the potential was."
The locker was used to store props and costumes for the school's drama department, according to Borean.
Borean estimated that damage to the locker was about $10,000, and about $10,000 to the contents as well.
No injuries were reported and no buildings were damaged.
San Mateo Union High School District Superintendent Dr. David Miller said Hillsdale High School is not open during the summer.