The fire, reported just before 10 p.m., quickly engulfed the seating area and sent a large plume of smoke into the air.
Crews were able to keep the blaze from spreading to the nearby gym, but the grandstands are a total loss.
No one was injured, but investigators are looking at whether Sunday's blaze is connected to another suspicious fire at the grandstands several weeks ago. That fire was quickly extinguished.
Firefighters were still on scene Monday morning watching for hotspots while investigators officials began trying to find out exactly how the fire started.
District Superintendent Dan Winter said insurance should cover the damage to the grandstands, and the school's homecoming game will be played at Aberdeen High School on September 29.
Other games may be played at Aberdeen, Hoquiam and Elma, but the exact schedule has not been determined.