All are members of the National Association of Schoolmasters Union of Women Teachers, whose general secretary, Nigel de Gruchy, will visit the school next week to lend support. Jack Duffy, the local secretary and one of the most prominent members in Scotland, teaches at Bannerman.
Mr Duncan is said to have acted "unprofessionally and inappropriately" in dealings with staff and union representatives, following a series of exchanges. The NAS accuses him of being anti-union, failing to follow procedures and ignoring letters and telephone calls. Mr Duncan has in turn accused the union of harassment.
The dispute, ironically, follows an attempt earlier this session by Mr Duncan to investigate a claim that two NAS members intimidated another member of staff, a charge they deny.
Mr Duncan sought to resolve the matter informally but was then accused of making "serious allegations" against the two without union representatives being present. The teachers have been placed under "immense stress", the NAS says.
The union also slates Mr Duncan for making critical statements about the NAS and its members. In a further twist, it says two other members were forced out of the school through stress caused by the management style.
Mr Gardner was sucked in after defending the head. Neither Mr Gardner nor Mr Duncan was available for comment.
Carol Fox, NAS Scottish official, said: "Our members have been left with no choice but to implement formal collective grievance procedures. We will not accept bullying and harassment."