Section 61. The school committees of two or more towns, each having a valuation less than two million five hundred thousand dollars, and having an aggregate maximum of seventy-five, and an aggregate minimum of twenty-five, schools, and the committees of four or more such towns, having said maximum but irrespective of said minimum, shall form a union for employing a superintendent of schools. A town whose valuation exceeds said amount may participate in such a union but otherwise subject to this section. Such a union shall not be dissolved except by vote of the school committees representing a majority of the participating towns with the consent of the department, nor by reason of any change in valuation or the number of schools.