California Code, Education Code - EDC § 44917

Except as provided in Sections 44888  1 and 44920, governing boards of school districts shall classify as substitute employees those persons employed in positions requiring certification qualifications, to fill positions of regularly employed persons absent from service.

After September 1 of any school year, the governing board of any school district may employ, for the remainder of the school year, in substitute status any otherwise qualified person who consents to be employed in a position for which no regular employee is available, including persons retired for service under the State Teachers' Retirement System. Inability to acquire the services of a qualified regular employee shall be demonstrated to the satisfaction of the Commission on Teacher Credentialing.

Any person employed for one complete school year as a temporary employee shall, if reemployed for the following school year in a position requiring certification qualifications, be classified by the governing board as a probationary employee and the previous year's employment as a temporary employee shall be deemed one year's employment as a probationary employee for purposes of acquiring permanent status.