Every district by its board, each year within 15 days after the close of its session as a board of equalization, shall levy an assessment sufficient to raise all of the following:

1. (a) Annual interest on any outstanding bonds of the district except to the extent that provision is otherwise made as permitted by law.
2. (b) Principal of outstanding bonds of the district as they mature except to the extent that provision is otherwise made as permitted by law.
3. (c) Obligations of the district due or to become due within the succeeding 12 months except to the extent that provision is otherwise made as permitted by law.
4. (d) A sum which with the other revenue of the district will meet the estimated current expenses of the district for the succeeding 12 months.