The board of supervisors of the county, the superintendent of schools of which has jurisdiction over any district, shall annually at the time of making the levy of taxes for county purposes, levy a tax for that year upon the property in the district for the interest and redemption of all outstanding bonds of the district. The tax shall not be less than sufficient to pay the interest on the bonds as it becomes due and to provide a sinking fund for the payment of the principal on or before maturity and may include an allowance for an annual reserve, established for the purpose of avoiding fluctuating tax levies. The tax shall be sufficient to provide funds for the payment of the interest on the bonds as it becomes due and also any part of the principal and interest that is to become due before the proceeds of a tax levied at the time for making the next general tax levy may be made available for the payment of the principal and interest.