27,000 secondary teachers are due to walk off the job today in a row over Junior Cert reform.
The one-day strike is going ahead despite an eleventh hour appeal by the Taoiseach yesterday, who told unions the compromise on the table is reasonable and should be accepted.
The ASTI and TUI are unhappy with plans to make them assess 40% of the work of their own students.
Teachers in Dublin said they didn't want to be picketing - they were losing a day's pay, and had tried other methods of protest.
Today's industrial action follows a similar work stoppage in December, and there could be at least one more strike in the weeks ahead.
350,000 pupils will be forced to stay at home later, but the TUI's President, Gerry Quinn, says they have their students' best interests at heart:
"It's out of concern for the wellbeing of the educational system [and] students," he said.
"We have exhausted avenues in order to try and secure an agreement, we've been frustrated in our attempts to try to protect this very important issue, and we do regret it has come to this - but we want this to come to a head."
He said he doesn't see the "reasonable compromise" that Enda Kenny said has been offered.