RCW 46.04.521: School bus.

School bus means every motor vehicle used regularly to transport children to and from school or in connection with school activities, which is subject to the requirements set forth in the most recent edition of "Specifications for School Buses" published by the state superintendent of public instruction, but does not include buses operated by common carriers in urban transportation of school children or private carrier buses operated as school buses in the transportation of children to and from private schools or school activities.