Cal. Education Code § 22808

A member of the Defined Benefit Program shall not be required to pay contributions under this part to receive credit for service under Section 22805 under any of the following conditions: (a) The service was performed after September 15, 1940, and the member returned to employment subject to coverage under the Defined Benefit Program prior to March 19, 1948. (b) The service was performed prior to January 1, 1950, and the member was continuously performing the service prior to that date and returned to employment subject to coverage under the Defined Benefit Program within six months following the termination of the service. (c) The service was performed prior to September 14, 1978, and the member entered that service after December 31, 1949, and returned to employment subject to coverage under the Defined Benefit Program within six months following the termination of the service. (d) The service was performed prior to January 1, 1992, and the member entered that service after August 1, 1990, and retired or returned to employment subject to coverage under the Defined Benefit Program and earned additional service credit within six months following the termination of that service or within six months after the completion of any period of rehabilitation offered by the United States government, excluding rehabilitation solely for educational purposes. Notwithstanding Section 22250, 22251, or 22253, employers of members subject to this section shall not be required to make the contributions required by Chapter 16 (commencing with Section 22950).