Except as otherwise expressly provided, if a single school district establish a high school, the board of trustees of such district shall be the high school board of trustees; and if any two or more districts establish a high school, the board of trustees of the district wherein the high school is located, together with the chairman of each of the cooperating districts, shall constitute the high school board of trustees. And except as otherwise expressly provided, if three or more adjoining school districts, none of which contains an incorporated town of twenty-five hundred inhabitants according to the last preceding census, shall cooperate to establish a centralized high school, the chairmen of the several cooperating districts shall constitute the board of trustees for the centralized high school.