Supervision of all matters pertaining to instruction in all the schools, under the direction and subject to the approval of the board of public education, shall be vested in the superintendent of schools, associate superintendents, assistant district superintendents, and school principals. Associate and assistant district superintendents may be appointed by the board of school directors, upon the nomination of the superintendent of schools. They shall receive such compensation as the board of public education may determine. They shall be under the supervision and direction of the superintendent of schools. Assistant district superintendents shall be assigned by the superintendent of schools to administrative districts. They shall, in conjunction with the school principals, inquire into and supervise all matters relating to the government, courses of study, method of teaching, discipline, and conduct of all schools in their respective districts, and shall report the same when required to the superintendent of schools.