Where common school lands have been valued under Section 15-10 and have remained unsold for 2 years after having been offered for sale in conformity to this Article, the trustees of schools or township land commissioners where the lands are situated may, without any petition having been filed, vacate the valuation thereof by an order entered in Book A of the county superintendent, and make a new valuation. They shall make a new valuation in the same manner as the former, and shall deliver to the county superintendent a plat of the land at such new valuation with the order of vacation, whereupon the county superintendent shall offer the land for sale as if no former valuation has been made.