The members appointed or elected in accordance with this section to the governing body shall be in addition to any other members prescribed by the Act establishing such school. In the year nineteen hundred and fifteen they shall be appointed or elected before the thirty-first day of August, and shall hold office until the first day of August, nineteen hundred and seventeen. In every second year after the year nineteen hundred and fifteen such members shall be appointed or elected in the month of July, and shall take office on the first day of August next ensuing their appointment or election, and shall hold office for two years or until the appointment or election of their successors. A casual vacancy shall be filled by appointment or election within fifty days of the occurrence of such casual vacancy.”