Teachers in Northern Ireland began industrial action this week in protest at a decision not to backdate performance pay.
Four unions representing around 20,000 members - the vast majority of classroom teachers in the province - are working to rule this term.
They are angered by the refusal of the Government and the employers to allow Northern Irish teachers, who reached level one of the upper pay scale in 2000, to apply for level two in September 2002 like their English counterparts.
The employers have agreed to let them apply for the extra pay from September 2003 but will not backdate any rise to the previous year, saying that funds are not available.
"It's a breach of faith," said Elza Margrain, regional organiser for the National Association of Schoolmasters Union of Women Teachers, the biggest teaching association in Northern Ireland. She said unions would fight to retain pay parity with their English counterparts.
The other unions involved are the Ulster Teachers' Union, the Irish National Teachers Organisation and the Association of Teachers and Lecturers. All four are taking slightly different action. Mrs Margrain said the variation would mean it had more impact.
The NASUWT action means teachers will not carry out around 17 tasks, many covered by the English and Welsh teacher workload agreement. But it did not involve boycotting training days. But as INTO was boycotting training days then they would not take place anyway, she said.
The work-to-rule represents the first industrial action taken by ATL members in Northern Ireland. Gerald Imison, deputy general secretary, said there was a clear injustice in pay arrangements that left them worse off than colleagues in England and Wales.