Wash. Rev. Code § 28A.400.260
Notwithstanding any other provision of law, any school district shall have the authority to provide for all employees within an employment classification pension benefits or annuity benefits as may already be established and in effect by other employers of a similar classification of employees, and payment therefor may be made by making contributions to such pension plans or funds already established and in effect by the other employers and in which the school district is permitted to participate for such particular classifications of its employees by the trustees or other persons responsible for the administration of such established plans or funds.
Notwithstanding provisions of RCW 41.40.023(4), the coverage under such private plan shall not exclude such employees from simultaneous coverage under the Washington public employees' retirement system.
RCW 28A.400.260