The amount to be raised for teacher salaries and contingent expenses must be such only as together with the public money coming to the city from any source is sufficient to establish and maintain efficient and proper schools for students in the city. The tax for purchasing, leasing, or improving sites and the building, purchasing, leasing, enlarging, altering, and repairing of schools may not exceed in any one year fifteen mills on the taxable valuation of property in the school district. The board of education may borrow, and when necessary shall borrow, in anticipation of the taxes to be raised, levied, and collected.