The total amount of temporary indebtedness incurred in any school district for the purpose of permanent improvements and the acquiring of school buses except obligations incurred under the provisions of section six hundred forty of this act,¹ shall never exceed five (5) per centum of the last assessed valuation of property taxable for school purposes therein. Except where such temporary indebtedness is to be refunded by the issue of bonds, as hereinbefore authorized, at or before the time of incurring such indebtedness for such purpose, provision shall be made for the collection of an annual tax, sufficient to pay the interest and also the principal thereof within the term of such indebtedness, as provided by law.