A quarter of secondary school parents say they've had to take out a loan or borrow from friends in order to meet back to school costs this year.
Research from Barnardos shows the average cost of sending a fourth class pupil to school is €320, rising to €972 for a student entering secondary school.
The charity's Back to School Survey, of over a thousand parents, has found half at primary level and two thirds at second level are worried about meeting the costs this year.
Barnardos is calling for the free book scheme to be extended to secondary schools.
CEO of Barnardos, Suzanne Connolly, says many families also struggle with the cost of uniforms.
"One very easy thing for schools to do, which won't cost them anything, is that they will ensure that there's a low cost, uniform option available for parents.
"And that is just implementing the 2017 circular the Department of Education put out to all schools asking them to reduce school costs for parents.
"Some of the parents we spoke to spend their summer worrying about the cost of sending their children back to school.
"What we're doing is we're calling on the government to extend the free school books to secondary schools, to ensure that all schools provide a low cost school uniform option, to end voluntary contributions, and to maintain the increase in the back to school allowance."