It would have been "catastrophic" for education if there had been no post-McCrone agreement, the EIS says. Ronnie Smith, the union's general secretary, said that the deal had bought three years of peace over pay and called on local authorities and ministers to honour it.
Staff were clamouring to join the chartered teachers programme and had shown "a real appetite" to develop professionally.
Mr Smith rejected criticism of job-sizing by headteachers' associations and other unions. All six teachers' associations had signed up to the deal on promoted posts, knowing what would be ahead. It was all too easy to be populist, he said.
"I do not disagree it has been a difficult exercise but it has been contaminated by the restructuring of a number of local authorities," Mr Smith said.
He doubted whether authorities which had moved to collective principal teacher posts could introduce curriculum reforms such as Higher Still under their revised structures.
It had taken time for some to realise that there would be differences in the size of jobs held by principal teachers "within schools and between schools".
Mr Smith said: "When you sign an agreement, you adhere to it. It's a table d'hote, it's not ... la carte. It's not pick and choose."