The head of the Teachers Union of Ireland (TUI) says it is 'very hard to predict' if schools will re-open as planned on Thursday.
It comes as education officials, union representatives and management bodies meet on Tuesday to discuss the re-opening plans.
Michael Gillespie, general-secretary of the TUI, told Newstalk Breakfast they need more details around isolation periods.
"There's a lack of clarity now as to how many days they need to isolate: is it from their PCR, is it from their antigen test?
"That will have a big effect on the schools opening on Thursday - so we need that sort of certainty.
"We also need school guidelines to issue today about school autonomy, because we know that schools are going to be different.
"There's going to be difference in different areas where there's going to be staffing issues.
"We want to prioritise the exam years, obviously, special needs classes - make sure they're able to return if there are staffing issues in schools".
He also says issues around investment in ventilation is "not just about 2021 - that's about 2022, 2023 so that we have these comfortable classrooms that are well-ventilated".
'Not going to be one size fits all'
On suggestions of a staggered return, as proposed by the ASTI, he says it is too soon to know.
"I think it's too early to say - I think that's why I said we called for autonomy of schools.
"It's not going to be one size fits all: we don't know how schools are going to be affected on Thursday.
"Over the next day or two... we're going to start finding out how schools are going to be affected; how many staff are not going to be there.
"We've now got texting and e-mail systems, so we'll be able to control who we can bring back on Thursday - if there is a need to reduce the number that are going to have to come back on Thursday because of the unavailability of teachers.
"Staffing is going to be big issue on Thursday."
Asked if he believes schools will re-open as planned on Thursday, he says: "I hope so, but it's very hard to predict with the Omicron.
"We don't know how many people are affected yet.
"But I would certainly hope they would be opening as near to full capacity as possible, given whatever staff are available".
While the ASTI has proposed a delayed and staggered re-opening of schools.
The union says it is "deeply concerned" that Education Minister Norma Foley may re-open them "without putting in place additional measures necessary to safeguard the health and safety of students and school staff".
"This would constitute an unacceptable risk in the context of the Omicron wave", the ASTI says.
It is calling for updated risk assessments to be presented prior to schools re-opening later this week.
"The priority must be that students and school staff can learn and work in an environment where there are appropriate safety measures in place to protect all concerned", ASTI president Eamon Dennehy says.