The ASTI has confirmed it will withdraw from substitution and supervision duties in secondary schools from Monday.
It means 400 schools are now unlikely to open after the midterm break.
It follows negotiations between the Department of Education and the teaching union at the Labour Court in long standing row over pay and conditions.
The ASTI says its members will turn up for teaching duties on Monday.
However, boards of management had previously warned they could not open if all duties - including substitution and supervision - are not covered, due to health and safety concerns.
The ASTI and Department of Education will hold further talks tomorrow.