The services described in Sections 1720, 1721 and 1722, except clerical, accounting, and stenographic services, shall be performed by persons who hold a valid credential, or a life diploma based thereon, authorizing administrative services.

Any person who is, and continuously since September 7, 1955, has been, employed in the office of a county superintendent of schools and is performing the services described in this article may continue to perform such services without possessing the credential otherwise required as long as he remains continuously employed in his position.