Mo. Rev. Stat. § 162.064

Each school district shall have on file a statement from a medical examiner which indicates that the driver is physically qualified to operate a school bus for the purpose of transporting pupils. Such statement shall be made on a biennial basis. The term "medical examiner" includes, but is not limited to, doctors of medicine, doctors of osteopathy, physician assistants, advanced practice nurses, and doctors of chiropractic. For new drivers, such statement shall be on file prior to the driver's initial operation of a school bus. This section shall apply to drivers employed by the school district or under contract with the school district.