The county superintendent of schools may, with the approval of the county board of education, employ personnel to supervise the attendance of pupils in elementary school districts under his jurisdiction which had less than 901 units of average daily attendance during the preceding fiscal year, in high school districts under his jurisdiction which had less than 301 units of average daily attendance during the preceding fiscal year, and in unified school districts under his jurisdiction which had less than 1,501 units of average daily attendance during the preceding fiscal year, if the districts are not served by any district supervisor of attendance.