Children from junior infants to second class will not have to observe social distancing but all others will have to be spaced a metre from their classmates.
€75 million is being allocated to allow schools to adapt their classrooms.
Yesterday it was announced that more than 1,000 extra teachers will be hired to help reduce class sizes as part of the Government's €376 million plan to reopen schools.
ASTI general secretary Kieran Christie says if schools do not have enough space they will have to look to the community for help.
"It might be necessary to approach the local community for their parish hall of their GAA facilities. Perhaps it is unfair to put pressure on them there by even mentioning that.
"Certainly though if the classes cannot be accommodated, other premises will have to be look at."
