The school committees of the cities or towns voting to unite for the purpose of the employment of a superintendent of the schools as prescribed in § 16-2-12, shall form a joint committee, for the purpose of this chapter; the joint committee shall be the agents of each city or town comprising the union. The school committees shall meet annually in joint convention at a place and time agreed upon by the chairpersons of the several school committees, and shall organize by the choice of a chairperson and secretary. They shall choose by ballot a superintendent of schools, fix his or her salary, and apportion the amount to be paid by each of the cities or towns, approximately according to the next preceding school census in the city or town. The union shall not be dissolved because the number of schools shall have increased beyond the number of sixty (60), nor, for any reason, for the period of three (3) years from the date of the formation of the union, except by concurrent votes of the school committees of a majority of the cities or towns constituting the union.