Mo. Rev. Stat. § 169.561
Any person retired and currently receiving a retirement allowance from either the public school retirement system of Missouri or the public education employee retirement system of Missouri, other than for disability, who elects to return to work in an employment capacity covered by either of the aforementioned retirement systems shall undertake such service under a new membership in the applicable system. The new membership for such a person shall have a vesting period of one year of creditable service, after which the person shall be eligible to retire and receive a second or subsequent retirement allowance for the service credit earned under the new membership in accordance with the law governing such matters. Contributions shall be made to the retirement system for any covered employment under the new membership at the same time and in the same manner as contributions are made for covered employment generally. Service credit shall be earned or may be acquired under the new membership in accordance with the law governing such matters. A second or subsequent membership or retirement allowance established or earned pursuant to this section shall be separate from and shall not be combined with any previous membership service credit or retirement allowance earned from the aforementioned retirement systems. Upon termination of covered employment under a second or subsequent membership, and in lieu of application for and receipt of a retirement allowance based on such service, the person may withdraw from the retirement system and receive a refund of the person's contributions during such membership in accordance with the law governing such matters; provided that, by so doing, the person shall forfeit any creditable service the person may have accrued under that membership. A person shall not receive a retirement allowance from the aforementioned retirement systems for any previous membership service while in covered employment under a new membership established pursuant to this section, nor shall a person receive such a retirement allowance in any month in which the person earns service credit under the new membership.
§ 169.561, RSMo