“County peace officer” shall also include probation officers, deputy and assistant probation officers, and persons employed in a juvenile hall or home and having as their primary duty and responsibility the counseling, supervision and custody of a group of youths assigned or committed to the hall or home. It shall also include persons employed as peace officers pursuant to Section 830.5 of the Penal Code, regardless of the administrative title of the position. It shall not include persons employed as teachers, instructors, psychologists, or to provide food,

maintenance, health or other supporting services even though responsibility for custody and control of youths may be incident to or imposed in connection with that service.

This section shall not apply to any contracting agency nor to the employees of a contracting agency until the agency elects to be subject to this section by amendment to its contract with the board, made as provided in Section 20474 or by express provision in its contract with the board.