Whenever any district acquires an operating property from any private person, firm, or corporation and a portion of the operating property is situated within the boundaries of any school district and at the time of such acquisition there is an outstanding bonded indebtedness of the school district, then the public utility district shall, in addition to the tax imposed by this chapter, pay directly to the school district a proportion of all subsequent payments by the school district of principal and interest on said bonded indebtedness, said additional payments to be computed and paid as follows: The amount of principal and interest required to be paid by the school district shall be multiplied by the percentage which the assessed value of the property acquired bore to the assessed value of the total property in the school district at the time of such acquisition. Such additional amounts shall be paid by the public utility district to the school district not less than fifteen days prior to the date that such principal and interest payments are required to be paid by the school district. In addition, any public utility district which acquires from any private person, firm, or corporation an operating property situated within a school district, is authorized to make voluntary payments to such school district for the use and benefit of the school district.