Except in a city and county, there shall be a county board of education, which shall consist of five or seven members to be determined by the county committee on school district organization. Each member of the board shall be an elector of the trustee area which he represents and shall be elected by the electors of the trustee area. In chartered counties the manner of selection of the county board of education shall be prescribed in the county charter, or by the county board of supervisors. In a county unified school district or in a unified or elementary school district which includes all of the territory over which a county superintendent of schools has jurisdiction, the governing board of the district shall serve as the county board of education.