Mo. Rev. Stat. § 177.161
In metropolitan school districts, all contracts for the erection of school buildings and all contracts for repairs and alterations or additions to school property or materials, equipment or apparatus to be furnished exceeding the sum of five thousand dollars shall be made by the board of education, after public letting, to the lowest responsible bidder complying with the terms of the letting. The necessary specifications and drawings shall be prepared for all such work, and bids therefor shall be solicited by advertisement as the board of education provides. No bid shall be entertained by the board of education which is not made in accordance with the specifications and drawings furnished and all contracts shall be let to the lowest responsible bidder complying with the terms of the letting. The board of education shall have the right to reject any and all bids. Contracts involving the expenditure of less than five thousand dollars for work to be done and materials or equipment to be furnished may be made directly by the superintendent of schools, or by the other officer or employee of the board of education that he designates with the approval of the board, without public letting and without advertising for or inviting bids. The board of education may, however, use its own employees to alter, maintain and repair school buildings, to maintain and repair apparatus or equipment, or to make improvements of school grounds without the letting of contracts whenever the total cost of labor on the job does not exceed the amount of twenty thousand dollars.
§ 177.161, RSMo