On the date appointed for the letting the board together with the superintendent of construction shall convene and let to the lowest responsible bidder, either as a whole or in sections as they may deem most advantageous for the district, the proposed work. No bid shall be entertained that exceeds the estimated cost unless for good and satisfactory reasons it shall be shown that the original estimate was erroneous. The board may reject all bids and advertise the work again if in their judgment the interest of the district will be served by doing so.