Officers swept the schools near Redmond, 16 miles east of Seattle, but did not find any more explosives, a spokeswoman for the King County sheriff's office said. The schools were closed and a nearby middle school was temporarily placed in lockdown.
The device was contained in a two-liter soda bottle left in the parking lot, the spokeswoman said, but she gave no other details about the nature of the device or how it was detonated.
A bus driver walking past the bottle bomb when it detonated sustained minor injuries and was being treated for exposure to fumes, according to a statement from Dr. Traci Pierce, superintendent of the Lake Washington School District.
No students were injured nor were any nearby when the blast occurred in the parking lot of the two schools - the Emily Dickinson Elementary School and the Explorer Community School, Pierce said.
Deputies from the bomb disposal unit had collected the item and would investigate, the sheriff's office said.