Teachers in the NASUWT union taking action short of a strike could have pay deducted for breaching their contract.
That is according to a letter from the teaching employers to school principals, seen by BBC News NI.
Members of the NASUWT in Northern Ireland began industrial action short of a strike in May.
The action affects a number of duties, including covering for other staff, overseeing exams, lesson plans, inspections and parent meetings.
