Colo. Rev. Stat. § 22-42-114
When approved at an election held pursuant to section 22-42-102, the board of education, from time to time, as the proceeds thereof shall be needed for the purposes specified in the notice of said bond election, shall issue bonds of the district in denominations of one thousand dollars or any multiple of one thousand dollars, in its discretion, bearing interest at a rate such that the annual and total repayment costs do not exceed the limits set forth in the notice of the bond election and payable at such time determined in the discretion of the board, which bonds shall mature serially, commencing not later than five years and extending not more than twenty-five years after the date thereof. Principal and interest thereon shall be payable at such place as shall be determined by said board and designated in said bonds. Said bonds shall be made callable for redemption, commencing no later than eleven years after their date, in such manner, with or without premium, as may be determined by the board. Interest on bonds issued on or after July 1, 1973, pursuant to this article shall be exempt from Colorado income tax.
C.R.S. § 22-42-114