Some 200,000 secondary pupils won't have school tomorrow - with no clarity yet on when classes will resume.
Talks between the ASTI and Department of Education officials are due to continue this afternoon - it's after four hours of negotiations yesterday - failed to reach a conclusion.
Both parties say they aim to resolve the long running dispute - ASTI members will turn up to teach tomorrow but won't carry out supervision and substitution duties - which means schools can't open due to health and safety rules.
Former Minister for Education Mary Hanafin says it's a concerning development: "When the teachers went on strike two weeks ago, it was clear cut.
"It was clean, they were on strike, schools were closed, that was one day.
"What is happening this week is actually much more difficult to manage, because what they are saying is we are available to teach, but we are not going to do substitution, we can't bring in people at short notice, we don't have people vetted so the schools are going to close indefinitely."