Firefighters responded to the blaze at about 11:30 p.m. after receiving an automatic alarm from the school.
They found a fire in a room on the second floor had activated the building's sprinkler system, which kept the blaze from spreading but failed to douse the flames.
Crews had difficulty entering the school because of several obstacles, including locked gates and doors, said Mike Proulx of the Renton Fire Department. Firefighters finally got in by breaking a window to get to the second floor and extinguished the blaze.
Proulx said fire, smoke and water caused about $40,ooo in damage to the building and $40,000 to its contents. Clean-up crews were called in to vacuum up water from several rooms.
Officials later said the fire was caused by an electrical malfunction in a fish tank - most likely the filter pump. Clean-up crews said the fish tank is completely black from the fire - but the fish survived.
The building was unoccupied at the time of the fire and no one was hurt.