Cal. EDC § 5090

Vacancies on school district governing boards or community college district boards are caused by any of the events specified in Section 1770 of the Government Code, or by a failure to elect. A vacancy resulting from resignation occurs when the written resignation is filed with the county superintendent of schools having jurisdiction over the district, except where a deferred effective date is specified in the resignation so filed, in which case the resignation shall become effective on that date. A written resignation, whether specifying a deferred effective date or otherwise, shall, upon being filed with the county superintendent of schools be irrevocable.