Whenever an order is entered establishing the district, the order shall finally and conclusively establish the regular organization and establishment of the district against all persons except the state of Missouri upon suit commenced by the attorney general within three months after the decree declaring the district established. The organization and establishment of the district shall not be directly or collaterally questioned in any suit, action or proceeding except as herein expressly authorized. This provision is understood not to apply to an election contest of the referendum, first board election, or any other election provided for in this chapter.