Notwithstanding Section 44850, and notwithstanding provisions of the charter of any city or city and county to the contrary, on and after July 1, 1978, the certificated employees of any school district governed by such a charter who serve as the head of a department of the district or in an administrative or supervisory position shall neither acquire nor retain permanent status in such position unless the employee is or becomes eligible for permanent status in accordance with provisions of this code.