The state agency, with the approval of the governor, is hereby authorized to modify at any time prior to January 1, 1974, the agreement of December 5, 1955, between the state and the secretary of health, education, and welfare, whereby under section 218 [42 U.S.C. § 418] the insurance system established by the Social Security Act was extended to services performed by individuals as employees of the state or any political subdivision thereof, to effect the exclusion of service performed in the employ of a school, college, or university if such service is performed by a student who is enrolled and is regularly attending classes at such school, college, or university.