The teachers union ASTI says only exam year students should return to school this week.
The union is calling for a staggered re-opening to schools on Thursday, amid concerns of staff shortages and inadequate ventilation.
It comes as education officials, union representatives and management bodies meet on Tuesday to discuss the re-opening plans.
ASTI president Eamon Dennehy told The Pat Kenny Show a lot more needs to be done.
"In its simplest form, I suppose prioritising - for one thing - the exam years, the Junior Certs and the Leaving Certs, and make sure that there's as little disruption to their education as possible.
"And of course a staggering then would mean that staff would be available for that number of students, and it would be possible maybe to have a more orderly start to the year".
He says they will bring this idea to the stakeholder meeting later on Tuesday.
"This is a proposal by us, and we want to table this idea and see what we can do with the other people involved in this decision-making process.
"I think it's a very big assumption now that schools will be able to open safely, and in an orderly fashion, Thursday or Friday or whenever they are about to open, without first of all considering the potential loss of personnel - the potential unavailability of teachers, etc."
He says safety is essential to the sustainability of education.
"We have to have the maximum possible safety procedures in place - and that is not the case yet.
"There are a lot of mitigations that could be done: improved masks, better ventilation, better filtering of air, etc.
"All these things would help to ensure that schools continue to function safely, and we're concerned about that".
He says a staggered return means more time for the other measures to be put in place.
And he adds while they will listen to the public health advice on schools, it needs to be explained.
"There's a mixed message here: one of the messages is that you have to be extremely, extremely careful not to meet anybody practically in order to avoid this extremely contagious virus.
"And at the same time over-crowded, badly-funded, badly-designed schools are safe.
"The obvious conclusion you have to draw from that is that they're not.
"We accept that this is an absolutely necessary service that we provide - but you can't provide it if you can't provide it safely.
"If you open schools and hope for the best, they probably definitely will close then".
Earlier the head of the Teachers Union of Ireland (TUI) said it was 'very hard to predict' if schools will re-open as planned.
Michael Gillespie told Newstalk: "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 believed schools will re-open as planned on Thursday, he Mr Gillespie said: "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".