The board of supervisors may, as preliminary to a sale, authorize the trustees of a township, where the sixteenth section or land selected in lieu of the sixteenth section has not been sold, to lay out the section into tracts as in their judgment will be for the best interests of the permanent school fund, conforming, as far as the interests of the fund will permit, to the legal subdivisions of the United States surveys, and appraise each tract at what they believe to be its true value, and certify to the board the divisions and appraisements made by them. The division and appraisement shall be approved or disapproved by the board at its first meeting after the report, and in case it disapproves, it may at once order another division and appraisement. If the board of supervisors approves, the county auditor shall make and keep a record of the division, appraisement, and approval; but school lands shall not be sold for less than the appraised value per acre, except as provided. A member of the board of supervisors, county auditor, township trustee, or a person who was engaged in the division and appraisement of the land, shall not be directly or indirectly interested in the purchase of the land; and any sale made, where the parties have an interest in the land, shall be void.