Colo. Const. art. IX, § 2

Establishment and maintenance of public schools. The general assembly shall, as soon as practicable, provide for the establishment and maintenance of a thorough and uniform system of free public schools throughout the state, wherein all residents of the state, between the ages of six and twenty-one years, may be educated gratuitously. One or more public schools shall be maintained in each school district within the state, at least three months in each year; any school district failing to have such school shall not be entitled to receive any portion of the school fund for that year.