In any county in which a special county tax shall have been voted under the constitution for such purpose or for school purposes generally, and in any school district in which a special district tax shall have been voted under the constitution for such purpose or for school purposes generally, the county board of education or the city board of education, as the case may be, with the approval of the State Superintendent of Education may issue and sell capital outlay warrants for the purposes of erecting, purchasing, altering, enlarging, improving, repairing and equipping school buildings and school playgrounds, and buildings for housing and repairing school buses, including sites for any such buildings and playgrounds; and for the purpose of purchasing school buses; and for the purpose of acquiring a school building already erected by another government body, which building is being transferred to the use and jurisdiction of the board issuing the warrants; or for any one or more of such purposes; issue and sell or exchange refunding warrants for the purpose of refunding any valid warrants heretofore or hereafter issued and constituting a preferred claim against the said tax, or, in the case of refunding warrants payable from the tax of a special school district which consists of a consolidation of two or more smaller special school districts, constituting a preferred claim against the tax of any of such smaller districts; provided, that the refunding warrants shall not be issued in an aggregate principal amount exceeding the sum of (i) the outstanding principal of such warrants being refunded, (ii) the interest accrued and unpaid thereon plus the interest to mature thereon until the date on which they are to be redeemed or paid, and (iii) the amount of any redemption premium required to be paid. Proceedings authorizing the issuance of refunding warrants under the provisions of this article shall identify the warrants being refunded, but no purchaser or holder of any such refunding warrant shall thereby be put upon inquiry or charged with notice of the nonexistence or invalidity of such refunded warrants, and the validity of such refunding warrants shall not be affected thereby. Warrants shall never be issued hereunder to an amount of principal and interest maturing in any fiscal year which, when added to the amount of principal and interest of all warrants then outstanding and constituting preferred claims against the said tax and maturing in said fiscal year, would exceed 80 percent of the annual proceeds of said tax, computed upon the basis of the last assessed valuation on which taxes were due and payable, of the county or of the district, as the case may be, as certified by the county tax assessor.