Classes of 35 and over are a cause of "serious concern" and were top of the agenda after teachers' salaries at yesterday's INTO conference in Galway.
"I have always taught in a class of 35 or more," Ms Angela Griffin, from the union's Dublin City South branch said. For the 27 teachers in her school, "it is a feat of superhuman proportions to teach the curriculum as we know it," she said.
"When children start school, it is their first major contact with society. What do we offer them? Reception classes of 35."
Ms Griffin said "I want to sit them around me and bring them into the world of make-believe when I read them a story. I want to reach every child in my class . . . don't tell me that I'm living in a world of make-believe myself by striving for an ideal." Large classes militate against this ideal, she said.
"It is obvious, despite the recent improvements, that class size remains an issue of serious concern," Mr Donal O Loingsigh, a member of the union's central executive council, said.
In particular, teaching in schools which are in designated disadvantaged areas where teachers continue to have classes of over 30, is "extremely demanding and difficult", he said, because of the "multiplicity of difficulties that go hand-in-hand with socio-economic deprivation".
Teachers who do not want to work in such areas cannot be criticised, but "we now have the critical situation of many unfilled permanent jobs in these schools", he said. "We are the only country in Europe to have such large infant classes."
Ms Maire Ni Chuinneagain, a member of the INTO's central executive committee, said it was time to reduce reception classes to no more than 20. The situation is compounded for those infants who are in a multi-class situation, she said.
Delegates want such multi-classes (which are largely in small schools where there are fewer teachers) to be no larger than 25 and they want single classes to be reduced to no larger than 30.
Ms Kathy McHugh, from the Tallaght branch, who has taught classes of more than 30 for more than 20 years, also "vehemently supported the motion".