Because of consolidations of school districts and for other reasons, the debt service millage voted by a school district for the payment of its outstanding indebtedness frequently provided a substantial surplus over the amount of the annual principal and interest requirements. This surplus may be used by the district for the purpose of paying the principal and interest of subsequent indebtedness incurred by it and may be pledged for that purpose or any other school purpose, provided that the district is in compliance with the uniform rate of tax.