The ASTI is calling on the government to urgently address the issue of overcrowding in classrooms.
The teachers' union says learning opportunities are being restricted because there is not enough space being made available to students.
It comes as the Joint Oireachtas Committee on Education and Skills holds the first of a two day sitting today, to discuss the challenges facing Ireland's school system.
Assistant general secretary of the ASTI, Moira Layden, says students are the ones losing out.
"One of the big this now, we all talk about active learning, putting students in groups around a table, getting them to correct each other's homework, getting them to talk each other through a problem if one doesn't understand it and the other does," she said.
"You can't do that when you have a classroom that's full."
Parental costs and principal pressures will also be examined today ahead of the new school term.
Parents, principals, teachers unions, management bodies and department officials will outline their concerns to the Education Committee's special two-day sitting.
"We are going to be talking about things like the issue of school places, the school accommodation crisis, PE pitches, for example, a wide range of issues," said Fianna Fáil's Education spokesperson Thomas Byrne.
"School costs for parents but also the cost of running schools for principals as well, and capitation.