Wash. Rev. Code § 28A.405.400
In addition to other deductions permitted by law, any person authorized to disburse funds in payment of salaries or wages to employees of school districts, upon written request of at least ten percent of the employees, shall make deductions as they authorize, subject to the limitations of district equipment or personnel. Any person authorized to disburse funds shall not be required to make other deductions for employees if fewer than ten percent of the employees make the request for the same payee. Moneys so deducted shall be paid or applied monthly by the school district for the purposes specified by the employee. The employer may not derive any financial benefit from such deductions. A deduction authorized before July 28, 1991, shall be subject to the law in effect at the time the deduction was authorized.
RCW 28A.405.400