Contracts for labor or materials entering into the construction of any improvement authorized by the district shall be awarded at public bidding except as herein otherwise provided. A notice calling for sealed proposals shall be published in such newspaper or newspapers of such general circulation as the board shall designate for a period of not less than two weeks (three issues) prior to the date of the opening of the bids. Such proposals shall be accompanied by a certified check for such amount as the board shall decide upon to guarantee compliance with the bid, and shall be opened in public at the time and place designated in the notice. The contract shall be awarded to the lowest and best responsible bidder; PROVIDED, That the board shall have authority to reject any and all bids.