Mo. Rev. Stat. § 169.430

All persons who shall hereafter become employees shall become members as a condition of their employment and shall receive no pension or retirement benefit from any pension or retirement system other than the retirement system established pursuant to sections 169.410 to 169.540 because of credited years of service in the school district, nor shall they be required to make contributions under any other pension or retirement system of any school district or state because of such years, except that this section does not prohibit the extension of the benefits and liabilities of Title II of the Social Security Act of the United States (42 U.S.C.A. Section 401 et seq.) to the employees of the school district for the purpose of supplementing the benefits provided by this law, through agreement by the district and the state pursuant to sections 105.300 to 105.440 *.