The legislature shall make such further provision by taxation or otherwise, as with the income arising from the general school fund will create and maintain a thorough and efficient system of public schools, adequate to the proper instruction of all youth of the state, between the ages of six and twenty-one years, free of charge; and in view of such provision so made, the legislature shall require that every child of sufficient physical and mental ability shall attend a public school during the period between six and eighteen years for a time equivalent to three years, unless educated by other means.