St Vincent de Paul has said there is enormous pressure on parents to pay voluntary secondary school charges which can range from €30 to over €550 per child.
An online SVP survey of almost 1,500 parents revealed the charges have a huge impact on family life.
Research and policy officer with the charity Niamh Dalziel said some parents felt the charge was not optional.
Speaking to RTÉ's Morning Ireland, Ms Dalziel said: "They (parents) talked hugely about the impact and how it affected their family's lives, and paying for those basic necessities or foregoing, putting off bills to be able to pay the charge to be able to do the best for their children."
She called on the Department of Education to look at the grant funding it gives to schools.
Ms Dalziel said grant funding is needed to serve its purpose and it currently does not appear to do that.
Schools that were contacted as part of the research said principals were conscious of the pressure the charges put on parents.
Ms Dalziel said the money was used for a variety of purposes including expenditure for lockers, journals and insurance.
Some parents gave testimonies to say their children could not access these items or were denied them when the payment was not made, she said.
Ms Dalziel said the costs of transition year are considerably higher for parents and students and the funding for this year does not "meet what it's supposed to do".
This can often lead to families deciding that students should not participate in transition year, she added.
