Gov. Code, § 3545

(a) In each case where the appropriateness of the unit is an issue, the board shall decide the question on the basis of the community of interest between and among the employees and their established practices including, among other things, the extent to which such employees belong to the same employee organization, and the effect of the size of the unit on the efficient operation of the school district.

(b) In all cases:

(1) A negotiating unit that includes classroom teachers shall not be appropriate unless it at least includes all of the classroom teachers employed by the public school employer, except management employees, supervisory employees, and confidential employees.

(2) Except as provided in subdivision (c), a negotiating unit of supervisory employees shall not be appropriate unless it includes all supervisory employees employed by the district and shall not be represented by the same employee organization as employees whom the supervisory employees supervise.

(3) Classified employees and certificated employees shall not be included in the same negotiating unit.

(c) In the case of a district which employs 20 or more supervisory peace officer employees, a negotiating unit of supervisory employees shall be appropriate if it includes any of the following:

(1) All supervisory nonpeace officer employees employed by the district and all supervisory peace officer employees employed by the district.

(2) All supervisory nonpeace officer employees employed by the district, exclusively.

(3) All supervisory peace officer employees employed by the district, exclusively.

A negotiating unit of supervisory employees shall not be represented by the same employee organization as employees whom the supervisory employees supervise.