In the event that an agreement for social security under the provisions of section 218(d)(3) of the federal Social Security Act is made applicable to services performed by employees in positions covered by the school employees retirement system and to services performed by employees who have elected under the provisions of section 79-988 as such section existed immediately prior to February 20, 1982, to remain under a preexisting system, such agreement shall also be made applicable to services performed by individuals as employees of the school district in positions not so covered, but which are otherwise eligible to the benefits of old age and survivors insurance under the provisions of section 218 of the federal Social Security Act as amended, and such employees shall be included in the coverage group, specified in such agreement.