The members to be appointed by the Timaru High School Old Boys’ Association and the Timaru High School Old Girls’ Association under paragraphs (f) and (g) of subsection one of this section shall in each case be appointed by a resolution of the appointing Association; and the first such appointments shall be made before the thirty-first day of March, nineteen hundred and fifty-nine, and subsequent appointments shall be made in the month of July in every second year thereafter, and all such members shall, unless sooner vacating office, hold office until their successors are appointed. Whenever a casual vacancy occurs among the members so appointed, the appointment of a member to fill that casual vacancy shall, within fifty days after the vacancy occurs, be made in a similar manner to that hereinbefore provided for an ordinary vacancy, and the member so appointed shall hold office only for the residue of the term of the vacating member.”