In the event any portion of such property shall be situated in any school district which, at the time of acquisition, has an outstanding bonded indebtedness, the city or town may in addition to the payments authorized in RCW 35.21.430, make annual payments to such school district which shall be applied to the retirement of the principal and interest of such bonds. Such payments shall be computed in the proportion which the assessed valuation of utility property so acquired shall bear to the total assessed valuation of the district at the time of the acquisition.