Sec. 9. CONTRACTS AND PURCHASES. The district trustees shall make all purchases of equipment and supplies for the various school districts and shall contract for all buildings and improvements and repairs and all other expenditures, but where the consideration involved is more than $100.00 such contracts and purchases shall be approved by the County Superintendent. The County Board of Education may, if so authorized in writing by the district trustees, purchase supplies and equipment for all the school districts in wholesale lots, upon requisitions made by the district trustees of the various schools.