Mo. Rev. Stat. § 162.351: Election commissioners to conduct elections in certain districts.

In any urban school district in a city having a population of more than three hundred thousand or in any seven-director school district located wholly within a city having a population of more than two hundred thousand and less than seven hundred thousand, or in any seven-director school district in a county having a population of more than seven hundred thousand the election authority in which the district is located shall conduct any or all school elections held in the district.