In each case where the question is in issue, the secretary shall decide, on the basis of the community of interest between and among the professional employees of the board of education, the wishes of the professional employees and/or the established practices among the professional employees including, among other things, the extent to which such professional employees have joined a professional employees’ organization, whether the unit appropriate for the purposes of professional negotiation shall consist of all persons employed by the board of education who are engaged in teaching or performing other duties of an educational nature, or some subdivision thereof, except that a unit including classroom teachers shall not be appropriate unless it includes all such teachers employed by the board of education.