Cal. Education Code § 44921

Notwithstanding the provisions of Section 44919, the governing board of a unified or high school district may employ persons requiring certification qualifications for the first semester only, and classify such persons as temporary employees, whenever the district expects a reduction in student enrollment during the second semester due to students completing graduation requirements at midyear. The number of persons so employed shall be limited to requirements based on the anticipated reduced enrollment as determined by the governing board. Persons so employed who are continued in employment beyond the first semester shall be deemed probationary employees for the entire school year. Any such employee shall be reemployed for the following semester or school year to fill any vacant positions in the school district for which the employee is certified. Preference for available positions shall be determined by the governing board using the method prescribed in Sections 44845 and 44846. 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.