The Association of Secondary Teachers Ireland has said it has a "substantial issue" with plans for teachers to assess their own students at Leaving Certificate level.
The reforms announced by Minister for Education Norma Foley envisages a significant reduction in reliance on final examinations, and the introduction of teacher-based assessment at Senior Cycle.
The reforms are to be phased in gradually over a number of years.
General secretary Kieran Christie said teacher assessment could be a "sticking point", but he said the union welcomed other aspects of today's announcement and would now consult with members.
ASTI President Eamon Dennehy said any changes to the Senior Cycle must be built upon fairness for students and trust in the system.
"Changes to education policy can have far reaching consequences for students, the education system and for society," Mr Dennehy said.
"The publication of the Chief Inspector's report last week once again confirmed that we have a high-performing and trusted second-level education system.
"It is longstanding ASTI policy that certification in the state exams is entirely externally assessed. This must be retained in all aspects of the development of the Leaving Cert. It is vital that the integrity of the state exams system is maintained."
Mr Dennehy added that it is essential that the "mistakes made" in the introduction of the Framework for Junior Cycle in recent years are not repeated, saying that those moves "sidelined the voice of teachers".
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Meanwhile, General Secretary TUI Michael Gillespie has concerns regarding teachers marking their own students in State examinations.
Speaking to RTÉ's Drivetime, he said: "There's a relationship between the teacher and the student that's fundamental here in Ireland, and it's worked really, really well.
"We have the fourth highest participation at the Leaving Cert in the OECD."
Mr Gillespie said that this high standard "happened because of the dedication of teachers, parents and students to the Leaving Cert," adding that there are "things we need to protect in it as well".
President of the Irish Second-Level Students' Union, Emer Neville, described the proposed Leaving Certificate reforms as "fantastic".
Speaking to the same programme, Ms Neville said: "We really welcomed them and we think it's fantastic that this is finally being proposed."
She said that reducing student stress is at the forefront.
Continuous assessment allows students to demonstrate their progression through their education, she added.
Sinn Féin's spokesperson on Education Donnchadh Ó Laoghaire welcomed the announcement.
However, Mr Ó Laoghaire said: "We need more detail from the Minister on what teacher assessed but externally moderated continuous assessment will look like."
He added: "It is important that the relationship between teacher and student, that is at the heart of the Irish school system, is preserved."
Social Democrats Education Spokesperson Gary Gannon also welcomed the plans but said they were long overdue.
Mr Gannon said: "Ultimately, we need to ensure that equity is at the heart of these changes and that we are truly reducing the stress caused to students by the Leaving Cert – and not just moving stress around."
