California Code, Education Code - EDC § 38112

(a) Except as provided in subdivision (b), the governing board of any school district may purchase any necessary school supplies and equipment, including standard school supplies and equipment listed by the county board of education, in the manner provided in this chapter, or the governing board of any school district may purchase such supplies and equipment directly from the vendor. Such direct purchase may be as a single district or two or more districts acting as a cooperative.

(b) An elementary school district having an average daily attendance of less than 2,500 during the preceding fiscal year may purchase standard school supplies and equipment directly from a vendor only by means of a purchasing cooperative representing a total average daily attendance in excess of 2,500 and then only if the county superintendent of schools has on file a document certifying the school district's membership in such a cooperative.