Primary heads in the Association of Head Teachers in Scotland are poised to join them in the boycott of the teachers' side where the Educational Institute of Scotland dominates discussion.
Ministers had promised in May to ensure a place for the heads' voice in the SNCT but nothing has happened yet to offer them any comfort.
George Ross, general secretary of the Headteachers' Association of Scotland, said the decision was based on three years of marginalisation.
But Mr Ross added: "The association remains open to discussing options for appropriate involvement in the SNCT in the future."
Local authorities as employers of heads are not keen to have them on the management side.
The first shots against EIS dominance have been fired by the heads but the other three teacher unions are similarly enraged and are close to forming an alliance that has been sparked by the outcome of the job-sizing exercise.
The HAS again fulminated against job-sizing and warned Peter Peacock, Education Minister, that his ambitions for schools could be undermined.
Alex Easton, headteacher of Falkirk High and the association's new president, blamed a flawed design brief for the outcome. "We best understand the complexities of senior management in secondary schools and how their potential may be fulfilled," Mr Easton said, yet the association was not consulted.
Mr Peacock, however, dismissed any further review of the job-sizing toolkit. "There is no going back," he said.