The government and control of county agricultural high schools in any county shall be vested in a board of six trustees, one from each supervisor’s district, three of whom shall be elected by the county board of supervisors and two of whom shall be elected by the county board of education. The county superintendent of education, who shall be the secretary of the board and whose term shall be his or her tenure of office, shall be the sixth member and shall represent the county at large. All regular terms shall be for a term of four years.