Whenever, by the returns of such election, it shall appear that a majority of the votes cast is in favor of making valid the increased debt, such debt shall thereupon become valid and of binding force; but the proper authorities in said school district shall, before issuing any obligation therefor, assess and levy an annual tax, which tax shall be equal to at least eight per centum of the amount of such increased debt and which shall be sufficient for and be applied exclusively to the payment of the interest and the principal of such debt within a period not exceeding thirty years from the date of such increase, and the moneys arising from such tax shall be applied annually, as far as the same may accumulate, to the redemption at par of the said outstanding obligation.