California Code, Education Code - EDC § 44920

Notwithstanding the provisions of Sections 44917 and 44919, the governing board of a school district may employ as a teacher, for a complete school year, but not less than one semester during a school year unless the date of rendering first paid service begins during the second semester and prior to March 15th, any person holding appropriate certification documents, and may classify such person as a temporary employee.  The employment of such persons shall be based upon the need for additional certificated employees during a particular semester or year because a certificated employee has been granted leave for a semester or year, or is experiencing long-term illness, and shall be limited, in number of persons so employed, to that need, as determined by the governing board.

Any person employed for one complete school year as a temporary employee shall, if reemployed for the following school year in a vacant 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.

For purposes of this section “vacant position” means a position in which the employee is qualified to serve and which is not filled by a permanent or probationary employee.  It shall not include a position which would be filled by a permanent or probationary employee except for the fact that such employee is on leave.