More than 27,000 secondary school teachers today forced the closure of 730 schools in a protest over proposed changes to the Junior Cycle education system.
With more than 350,000 students given the day off over the strike action, teachers insisted that they were doing it for the good of the education system.
The strike is the first day of action against changes that would see teachers assessing their own students in parts of their exams, which teachers claim will lead to inconsistencies in the marking of exams and falling standards as a result.
Outside St David's CBS in Artane, a group of teachers braved the cold with their placards to put their point across.
Martin McCarthy, a teacher at St David's CBS for 37 years, said he was not striking for his own aims, but those of students.
"The Junior Cert exam from my point of view as both a teacher and a parent works," he said.
"I had two of my children do the Junior Cert two years ago and I was very happy that the exam was impartial, and enabled me and my children to get a fair idea of where they stood," he added.
"It helped them make a proper selection for their Leaving Cert exams, so why change a system that works?" he asked.
Asked if he felt the teachers had public support in shutting schools for the day, Mr McCarthy said he thought they did.
"I'm not here for an increase in salary. I'm not here for an improvement in my conditions. I'm here because I believe that what the minister is doing is a huge mistake," he explained.
"I could just as easily say let the minister have her way, go ahead and have 100pc assessment, but if I were to do that I would definitely be doing a disservice to my students," said Mr McCarthy.
Teacher Jim O'Neill and ASTI committee representative said that they had taken a stand to "protect the integrity of the examination system".
"The dismantling of the examination system and the removal of the state exam commission from the system is not good and we disagree with that fundamentally," Mr O'Neill said.
"A State-marked and State- protected system means that there is a uniform standard across the country, and that the student doing an exam in our school is getting the same test as one in Galway or Donegal."
The new proposals would see 40pc of Junior Cert marks awarded through school-based continuous assessment with the remaining 60pc coming from the State exam.
Ms O'Sullivan said she had "moved 60pc" of the way to meet the teachers and believed the strikes could have been avoided if the unions had got involved in "meaningful" talks.
"I do think the strike is unnecessary," she said. "I do think it's disproportionate and I think it's disappointing that the schools are closed today."
Teachers' unions believe the new system will undermine the "credibility, transparency and fairness of the examinations process".
The president of the Teachers' Union of Ireland (TUI) Gerry Quinn said a system of teachers assessing their own students for State exams will "significantly change the relationship between the teacher and student".
"We agree with the minister that new modes of assessment such as practical work and portfolios are an important development. We support this," he said.
"However, we want all state exam components to be externally assessed. In recent months we have exhausted all available avenues in an effort to have our concerns addressed."
Jim O'Neill said teachers are not resisting all change in the education system, but only parts of it, and that they are already working with ongoing assessment of projects.
"We are implementing the syllabus and short courses, and the project work that is involved in subjects at Junior Cert level, but it's the examinations we think is the most important thing at the end of it," he said.
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