The principals' group warned some schools were seriously affected

The work-to-rule over a pay dispute has been going on for almost two months.

Teachers' unions voted to take industrial action because they say they are facing a shortfall in a pay increase, compared to colleagues in England and Wales.

The group, which represents principals said on Tuesday that the effect on some schools had been serious.

It said some parent-teacher meetings were being moved to school hours, after-school clubs were being cancelled and teachers were refusing to take part in training sessions.

Union spokesman Frank Bunting of the Irish National Teachers' Organisation said teachers had told him they were happy to opt out of regular duties.

"The action itself is extremely popular with teachers and indeed, many of our teachers have said that they don't wish to return to the additional bureaucracy of the duties they did before the industrial action," he said.

"We are saying to the management side that they must be very creative in bringing forward proposals."

Teaching employers have agreed to take part in arbitration.

BBC Northern Ireland education correspondent Maggie Taggart said this appeared to be a "slow process" with another meeting scheduled for next week.