Mo. Rev. Stat. § 178.300: Part-time schools established, when.

Whenever in any school district in this state not less than twenty-five employment certificates for children under sixteen years of age have been issued and are in full force and effect, the school district shall establish and maintain part-time schools, departments or classes for the employed children for not less than four hours per week and for a term not less than that in which schools are regularly in session in the district.