(3) employees of County Board of School Trustees and County School Superintendents (Section 17.91, Texas Education Code) shall be and are hereby transferred effective September 1, 1975, from the State Department of Public Welfare to the Employees Retirement System of Texas. All contracts and agreements in existence on the effective date of the transfer between the State Department of Public Welfare and the United States government or any and all local political subdivisions of the State of Texas or any other governmental entity shall remain in full force and effect and, upon validation by the Employees Retirement System of Texas, shall become effective contracts or agreements between the Employees Retirement System of Texas and such United States government or any agency thereof or any political subdivisions or other governmental entity of the State of Texas. The Employees Retirement System of Texas shall succeed to and be vested with all the rights, powers, duties, personnel, property records, trust funds, and appropriations now held by the State Department of Public Welfare for the operation and administration of the social security programs.