The INTO says any move to reopen schools has to be done in line with public health advice.
It's after Health Minister Simon Harris said it could start by bringing back students one day a week.
John Boyle, general secretary of the INTO, says it would mean huge numbers of people congregating in one place.
"I couldn't for the life of me, for example the school where I was most recently - 1,600 students, and maybe as many parents turning up on the day.
"It would have to be very very gradual.
"In some other jurisdictions they have been doing this - a small number of schools opened in Denmark last week, and we'd have to learn from the experiences there and any mistakes that they made."
