Cal. Gov. Code § 20513

Notwithstanding the election given in Section 20514 to contracting agencies, whether to subject themselves and their employees thereto, the contract of school districts in which the average daily attendance of all districts combined is in excess of 400,000 and which are governed by the same governing board, without action by the governing board, shall be subject to that section and shall include all provisions authorized by Section 20514 to be included therein by contract amendment.