CA Educ Code Section 45183

(a)

Assignment of duties to bargaining unit employees for which differential compensation is designated, other than a temporary assignment of less than 20 working days, shall be made on the basis of seniority among those employees within the appropriate class who request such an assignment.

(b)

This section may be waived by agreement between the governing board of a school district and the exclusive representative of the unit of classified employees to be affected by the waiver.