Mo. Rev. Stat. § 247.420
The board shall meet at the place to be designated by the board as often as the needs of the district require on notice to each member of the board. The board shall cause to be made an annual audit of the receipts and expenditures of the district, by a certified public accountant, the cost of said audit to be paid by the district. Four members of the board shall constitute a quorum at any meeting. Any vacancy on the board shall be filled by the remaining member or members of the board, the appointee or appointees to act until the next regular election at which directors are elected as herein provided, when vacancy or vacancies shall be filled by election. If the board shall fail, neglect or refuse to fill any vacancy within thirty days after the same occurs, the court having jurisdiction shall fill such vacancy.
§ 247.420, RSMo