The county superintendent of schools may, with the approval of the county board of education, employ one or more supervisors of health, as supervisors of health are defined in Section 49420, to provide health services to pupils in elementary school districts under his jurisdiction which had less than 901 units of average daily attendance during the preceding fiscal year, to pupils in high school districts under his jurisdiction which had less than 301 units of average daily attendance during the preceding fiscal year, and to pupils in unified school districts under his jurisdiction which had less than 1,501 units of average daily attendance during the preceding fiscal year.