Wash. Rev. Code § 28A.225.260
If the laws of another state permit its school districts to extend similar privileges to pupils resident in this state, the board of directors of any school district contiguous to a school district in such other state may make agreements with the officers of the school district of that state for the attendance of any pupils resident therein upon the payment of tuition.
If a district accepts out-of-state pupils whose resident district is contiguous to a Washington school district, such district shall charge and collect the cost for educating such pupils and shall not include such out-of-state pupils in the computation of the district's share of state and/or county funds.
The board of directors of any school district which is contiguous to a school district in another state may make agreements for and pay tuition for any children of their district desiring to attend school in the contiguous district of the other state. The tuition to be paid for the attendance of resident pupils in an out-of-state school as provided in this section shall be no greater than the cost of educating such elementary or secondary pupils, as the case may be, in the out-of-state educating district.
RCW 28A.225.260
Education of children with disabilities: RCW 28A.155.040.