Section 31. The school committee shall consist of the mayor, who shall be the chairman, and six members elected at large. At the first regular municipal election held in any city after its adoption of one of the plans provided in this chapter, there shall be elected, except as provided in this section, two members to serve for one year, two for two years and two for three years, and annually thereafter there shall be elected two members to serve for three years. If the plan adopted provides for elections to be held biennially, there shall, except under Plan E or F, be elected at the first regular municipal election held under the provisions of such plan, three members to serve for two years and three members to serve for four years, and biennially thereafter three members to serve for four years. After the adoption of Plan E by a city, the six members other than the mayor shall be elected at large for terms of two years each by proportional representation as hereinafter provided.