The fire forced the school to move classrooms elsewhere at the school, a school offiical said. It was the second fire in a classroom portable in a year.
"We had another portable that burned last june. We're still waiting on the district to repair that one," said Carmen Denhams, administrative assistant.
Oakland Fire Lt. David Brue said fire and police investigators are still trying to determine the cause of the fire. When firefighters arrived at 8:15 p.m. Thursday, they found the fire burning inside the portable.
Brue said the damage estimate included $30,000 to the structure and $10,000 to the contents.