Cal. Ed. Code § 19478
When any warrant is presented to the treasurer for payment and it is not paid for want of funds, the treasurer shall endorse thereon "not paid for want of funds" with the date of presentation and sign his name thereto, and from that time the warrant bears interest at the rate of 6 percent per annum until it is paid or until funds are available for its payment and the county treasurer gives notice to the warrant holder that funds are available for the payment. The giving of the notice is deemed complete upon deposit thereof in the United States mail in a sealed envelope addressed to the warrant holder at his address given by him at the time of presentation of the warrant to the treasurer, with postage thereon fully prepaid and registered.
Ca. Educ. Code § 19478