Bids for the work to be let under this chapter must be forwarded to the district office and must be sealed securely to prevent opening without detection. There must be endorsed upon the outside of the envelope containing the bid a statement of what work the proposal is for. The bids must be opened by the board at the expiration of the time limited in the advertisement for receiving the same, which may not be less than fifteen days after the first publication of the advertisement, or at such other time as the board may appoint. Only bids that are accompanied by the bond provided for in section 61-35-89 may be considered by the board.