Wash. Rev. Code § 28A.160.120
Any school district board of directors or any intermediate school district board may enter into agreements pursuant to chapter 39.34 RCW or chapter 35.58 RCW, as now or hereafter amended, with any city, town, county, metropolitan municipal corporation, and any federal or other state governmental entity, or any combination of the foregoing, for the purpose of providing for the transportation of students and/or members of the public through the use, in whole or part, of the school district's buses, transportation equipment and facilities, and employees: PROVIDED, That any agreement entered into for purposes of transportation pursuant to this section shall conform with the provisions of RCW 35.58.250 where applicable and shall provide for the reimbursement and payment to the school district of not less than the district's actual costs and the reasonable value of the use of the district's buses, and transportation equipment and supplies which are incurred and otherwise provided in connection with the transportation of members of the public or other noncommon school purposes: PROVIDED FURTHER, That wherever public transportation, or private transportation certified or licensed by the Washington utilities and transportation commission is not reasonably available, the school district or intermediate school district may transport members of the public so long as they are reimbursed for the cost of such transportation, and such transportation has been approved by any metropolitan municipal corporation performing public transportation pursuant to chapter 35.58 RCW in the area to be served by the district.
RCW 28A.160.120