The Association of Secondary Teachers of Ireland is joining the Teachers Union of Ireland in backing the predictive grades system, which will replace this year's Leaving Cert, but says it has huge concerns.
The union's recommending its members engage with calculated grades for students but it still has concerns about possible legal issues.
It is after the ASTI held lengthy talks on the issue late last night.
But General Secretary, Kieran Christie, says predictive grades are not an ideal replacement.
He said: "This is a less than perfect system.
"We have huge anxiety and trepidation about participating in it, but for good or ill, we've decided to do so.
"What we're saying is now that it's there and we're going to engage with it, we'll be insisting it is operated to the highest standard possible."
The ASTI said it wants students to progress to the next stage of their lives, so it is advising its members to engage with the Calculated Grades for the Leaving Certificate process.
It is after the Government decided to call off this year's written exams and replace them with calculated grades.
The ASTI said it has a long-standing policy of teachers not assessing their students for the purpose of State Certification.
However, it says in the context of the current global pandemic it recognises that this year it is necessary to engage with this new process.
The union said it has a number of major concerns with the process, which it is calling on the Minister for Education and Skills to address as a matter of urgency.
These include the Minister's rescinding of his previous decision to award students 100% for orals and certain practical examinations which were to have taken place earlier this year.