The fire had taken hold in one of the five or six classrooms in the block by the time the fire brigade arrived. The blaze had also spread to the loft.
By 6am on Monday firefighters were reporting that the fire was under control.
Fire engines from six stations had been called to the scene. The cause of the blaze is unknown, the northern Stockholm emergency services centre wrote in a press release on Monday morning.
Representatives from the emergency services and from Österåker council, which is responsible for the school, are due to meet the school’s management on Monday. The school plans to reopen the unaffected parts of the building when classes resume on Tuesday.