In addition to the authority otherwise provided in RCW
28A.160.010 through
28A.160.120 to school districts for the transportation of persons, whether school children, school personnel, or otherwise, any school district authorized to use school buses and drivers hired by the district for the transportation of school children to and from a school activity, along with such school employees as necessary for their supervision, shall, if such school activity be an interscholastic activity, be authorized to transport members of the general public to such event and utilize the school district's buses, transportation equipment and facilities, and employees therefor: PROVIDED, That provision shall be made for the reimbursement and payment to the school district by such members of the general public of not less than the district's actual costs and the reasonable value of the use of the district's buses and facilities provided in connection with such transportation: PROVIDED FURTHER, That wherever private transportation certified or licensed by the utilities and transportation commission or public transportation is reasonably available, this section shall not apply.