Expectations need to be managed about the reopening of schools in September, according to the National Association of Head Teachers (NAHT).
Its NI vice president, Graham Gault, said comments by the First Minister Arlene Foster on Thursday were “unrealistic and undeliverable”.
Mrs Foster told BBC NI’s The View the executive wanted “to get everybody back to school in September”.
Mr Gault said that would need “very drastic changes”.
On Thursday, Education Minister Peter Weir announced that the 2m social distancing rule would be reduced to 1m between pupils to allow “full classes to attend” school as coronavirus lockdown restrictions are eased.
In a letter sent to principals, the minister said: “The 1m guidance between children is to be followed as far as possible within the confines of the physical capacity of each classroom and the 2m rule for staff adhered to fully.”
Peter Weir added, subject to ongoing review, the new term will start on 24 August for Primary Seven, Year 12 and Year 14 pupils.
Graham Gault, who is principal of Maghaberry Primary School, said he had measured his own classrooms and could manage 15 pupils rather than the usual 30.
“At the moment it looks likely, if nothing changes, that I’ll be able to welcome back half of my school cohort,” he told BBC News NI’s Good Morning Ulster.
Mr Gault said “great progress” had been made in recent days and trade unions have worked with the Department of Education to create “workable solutions” on how to manage school numbers safely.
However, in response to a suggestion by the first minister that schools should “find extra space”, such as assembly and dining halls, as well as “facilities beside the schools”, Mr Gault added the comments “stopped me from sleeping last night”.
“I wondered where I was going to be finding the teachers from to take a class of children into another building on up the road.
“There are so many implications to that,” he explained.
“It’s really important that our communities don’t listen to yesterday’s comments and think that children will be coming back to school as normal in September.
“Schools want our children to be back full-time, it’s what we crave, but that’s not going to happen unless something very drastic changes.”
By Robbie Meredith, BBC News NI Education Correspondent
There is a hope that as many children can go back to as many schools as possible, as often as they can, come September.
We’re not at the stage though of most schools being able to take pupils back full-time, I think everyone would admit that, most principals certainly.
Who knows where we might be in mid-August.
I understand that in the guidance that comes out today there will be that aspiration to get all children back to school full-time, but that part-time element has been flagged up.
The fact that primary school children will probably only be in school around two days or half of a week, that most post-primary children will be in one week in every two, that guidance is still in there, it hasn’t disappeared.
Julie Thomas, principal of Clandeboye Primary School in Bangor, told Good Morning Ulster the reduction of social distancing guidelines has welcomed, however “has not given us any more hope that we will be able to bring the entire school community back into our building”.
“It’s very important that at this point we do not overpromise the parents what we are going to be able to deliver once we enter the new academic year.”
Methodist College Belfast Principal Scott Naismith said: “The minister has been overly optimistic in his assessment that this will enable many schools to accommodate all pupils each day.”
He said “favourable pupil-staff ratios and accommodation” means Methody’s preparatory schools can “get all the pupils back all the time from August”, however, complexities in the senior school means “at best we will get half the school accommodated safely in line with the new rules”.