In any school district organized under the provisions of Section 35502 and which is also a high school district, and which districts are governed by a board of school trustees, and which districts have an average daily attendance in the elementary school district of at least 800, as shown by the last report of the principal of schools in the elementary school district, on file in the office of the county superintendent of schools, the trustees of the school district may appoint a clerk, who shall not be one of their own number, to act for the elementary district trustees and the high school district trustees to hold office at the pleasure of the board of trustees. The board may fix the salary of the clerk at a sum not exceeding twenty-five dollars ($25) per month for the two districts, which shall be paid in the same manner and from the same funds as other incidental expenses of the districts are paid.