California Code, Education Code - EDC § 44824

A full-time probationary or permanent classroom teacher currently employed by a school district which decides to maintain classes on Saturday or Sunday, or both, shall not, without his written consent, be required to teach under such program for more than 180 full days during a school year, or for more than the number of full days the schools of the district were maintained during the year preceding implementation of weekend classes, whichever is greater. This section shall not be construed as limiting the power of any governing board of a school district to govern the schools of the district, including the assignment of teachers employed by the district. No such classroom teacher shall be assigned to perform services on a Saturday or Sunday if such teacher objects in writing that such assignment would conflict with his or her religious beliefs or practices.