The board of supervisors of the county, the superintendent of schools which has jurisdiction over any school district, shall annually at the time of making the levy of taxes for county purposes estimate the amount of money required to meet the payment of the principal and interest on bonds of the district authorized by the electors of the district and not sold, and which the governing board of the district informs the board in their belief will be sold before the next tax levy, and the board of supervisors shall levy a tax sufficient to pay the principal and interest so estimated.