If the adjustment schools are organized by the joint action of two or more counties, the boards of supervisors of the counties may by concerted action by duly adopted resolutions entrust the government and management to a governing board, which shall be any of the following:
(a) The board of education of the county in which at least one adjustment school is located.
(b) The probation committee of the juvenile court of the county in which at least one adjustment school is located.
(c) A board of trustees composed of seven members who shall represent all of the counties and each of whom may be selected from either the county board of education or the probation committee of the juvenile court of his or her respective county as shall be determined in the joint resolutions of the boards of supervisors.