The next phase of reopening schools will not go ahead if there is a spike in COVID-19 cases, the INTO has warned.
Around 300,000 pupils in the first four years of primary school, as well as Leaving Cert students, will return to class tomorrow.
Fifth years and all other primary school classes will return to school from March 15th, with all outstanding classes to return after the Easter break.
The General Secretary of the INTO, John Boyle, says the national teachers' union will be keeping a close eye on reports detailing coronavirus cases in schools.
He told Newstalk Breakfast with Susan Keogh that teachers are looking forward to getting back into the classroom.
However, the return to school will be far from normal, he added.
It comes as the Deputy Chief Medical Officer appealed to parents to limit their interactions with other people when schools reopen.
Dr Ronan Glynn has urged parents not to congregate at school gates and to avoid organising play dates.
"I think the big change is that Dr Glynn wrote to all the parents of Ireland on Friday, you won't see too many parents near a school, you won't see adults congregating," Mr Boyle said.
"Anyone who does have to come to the school to drop off the children or pick them up will be wearing a mask."
NPHET has taken "a very cautious approach" to schools reopening, with the new variants of the virus often mentioned, Mr Boyle said.
"I sincerely hope that if there are some cases, and I suppose it will be inevitable there are some, I hope that parents do not send children in with symptoms, that's the starting point.
"We do know there will be a public health risk assessment carried out in every school where there's a confirmed case.
"Each school is different, we have small schools with only 12 children in the class, and then large schools with over 30 children in the class.
"I think the big difference from our point of view is we have advised our principal teachers that if you get that dreaded phone call that there was a case in your school, that rather than doing a quick public health risk assessment, you take the time to go to the source, talk to the staff member involved and get a detailed report.
"In that situation, if there is a positive test, say in a school or a class, we have this batch testing where the whole class is tested at the test centre, even if it's a Saturday or a Sunday, the test results come back really quickly.
"So we are confident there will be a more robust testing and tracing system."
'A hell of a lot' of factors
Whether schools will definitely reopen on March 15th depends on a number of factors for third to sixth class children, with reports sent every week to unions and stakeholders, Mr Boyle said.
"We'll be getting a report from the special classes and that's why we're not certain that schools will open on the 15th because it will depend on a hell of a lot of things for the third, fourth, fifth and sixth class children," he explained.
"But particularly how things go, and we'll be getting those reports every Monday or Tuesday.
"If the numbers spike in schools then obviously the second half of the reopening won't happen.
"So it really behoves everyone to cooperate and make sure schools do fully reopen in the middle of March."
Public health officials are aware that with the new variants, "you can't be taking any risks", he said.
Parents will also be required to return a form to the school attesting that their child is COVID-free, while more testing and tracing will be done.
A new aspect to the schools reopening is that teachers will be wearing "high-grade masks" which unions had called for last year.
He added that the communication from the Government on the PPE was "a bit late", with Cabinet only signing off on the measure last Tuesday.
"In the intervening days, we've heard that the portal where you draw down the equipment, it has quite a lot of those masks available," he said, with deliveries being made nationwide in the last few days.