Cal. Pub. Cont. Code § 20281

The purchase of all supplies, equipment and materials, when the expenditure required exceeds twenty-five thousand dollars ($25,000), and the construction of facilities and works, when the expenditure exceeds three thousand dollars ($3,000), shall be by contract let to the lowest responsible bidder. Notice requesting bids shall be published at least once in a newspaper of general circulation, which publication shall be made at least 10 days before bids are received. The board may reject any and all bids and readvertise in its discretion.