California Code, Education Code - EDC § 44914

If an employee of a school district has served as a probationary employee of the district in a position requiring certification qualifications, for one complete school year, and in the year immediately preceding the service as probationary employee has served as a substitute employee, or as a substitute and probationary employee, serving in both capacities during the same school year in the schools of the district, at least 75 percent of the number of days the regular schools of the district were maintained, the governing board of the district may count the year of employment as a substitute or as a substitute and probationary employee as one year of the probationary period which he is required by law to serve as a condition to being classified as a permanent employee of the district.