Within 30 days after an increase in the number of directors has been authorized, the board of directors shall appoint two qualified electors of the district to serve as members of the board until the next biennial election, at which there shall be elected two directors in addition to the number previously authorized. After the directors elected at the biennial election have qualified, they shall meet and classify themselves so that one shall hold office for two years and the other for four years. Thereafter, there shall be elected at each biennial district election a number of directors, corresponding to the number whose terms expire, who shall hold office for four years.