Mo. Rev. Stat. § 162.032
If a school district is annexed to an existing district or divided into two or more districts by a vote of the citizens, or is dissolved under the lapse procedures in section 162.081, court action, or any other authority of Missouri or federal laws, the successor school district shall become responsible for ensuring access to continuation of health insurance coverage for retired teachers and employees of the district if the original district offers health insurance coverage to its retirees at the time of its loss of corporate structure. If an original district is divided into multiple successor districts, such responsibility shall be assigned to the successor district with the largest eligible pupil count in the most recently completed school year.
§ 162.032, RSMo