RCW 28A.160.110: School buses, authorization for parent, guardian or custodian of a student to ride—Limitations.

Every school district board of directors may authorize any parent, guardian or custodian of a student enrolled in the district to ride a school bus or other student transportation vehicle at the request of school officials or employees designated by the board: PROVIDED, That excess seating space is available on the vehicle after the transportation needs of students have been met: PROVIDED FURTHER, That private or other public transportation of the parent, guardian or custodian is not reasonable in the board's judgment.