Ed. Code, § 24922

The plan may provide that the retirement salary shall be a stipulated monthly sum, or that all benefits under the retirement feature of the plan shall be based upon the monthly salary for each year of future active service in the district earned by the employee up to the date of retirement and upon the average monthly salary earned by the teacher or other eligible employee during the year immediately preceding the adoption of the plan and the number of years of past active service of the employee in the district, and subject to those provisions made in the plan for minimum benefits. No employee with less than 15 years’ service in the district prior to normal retirement age shall receive the benefit of the minimum. In no instance shall the retirement benefits be based upon or allowed for any amount of salary in excess of the sum of five hundred dollars ($500) per month.