Firefighters put out a fire shortly after 3 a.m. when an alarm went off at the school in the 11700 block of Meridian Avenue S.
"They extinguished a small fire burning at a door on the southwest corner of the building," Snohomish County Fire District 1 spokeswoman Leslie Hynes said. "The firefighters put the fire out before flames got inside the building."
Damage to the door and adjacent wall and ceiling was estimated at $3,000, Hynes said.
Firefighters also discovered someone had attempted to set another fire outside a nearby portable classroom.
One of the fires was set near a music room, Mukilteo School District spokesman Andy Muntz said.
There were no suspects Thursday morning.
The county fire marshal's office is investigating the case as arson.
Anyone with information about the fires can contact investigators at 425-388-3557.
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