Cal. Education Code § 44951

Unless a certificated employee holding a position requiring an administrative or supervisory credential is sent written notice deposited in the United States registered mail with postage prepaid and addressed to his or her last known address by March 15 that he or she may be released from his or her position for the following school year, or unless the signature of the employee is obtained by March 15 on the written notice that he or she may be released from his or her position for the following year, he or she shall be continued in the position. The provisions of this section do not apply to a certificated employee who holds a written contract with an expiration date beyond the current school year, or to a certificated employee holding a position that is funded for less than a school year, or to a certificated employee assigned to an acting position whose continuing right to hold this position depends on being selected from an eligible list established for the position, or to the termination of employment pursuant to Section 44955.