A report published by the CPAG in May estimated the minimum cost of education is more than £1,000 for primary school children and almost £2,300 for secondary school children, a rise of 30% on three years ago.
It said the cost of food for the school day and technology for learning were the main sources of increases for secondary school children.
"This is a really significant amount of money for families," said Ellie Harwood, senior education policy officer for CPAG.
"It can be a real struggle for households on low and middle incomes to meet the cost and obviously the more children you have the greater the cost".
Ms Harwood said research had suggested uniform costs had "flat-lined", which she hoped was a "reflection in changes of school policy".
At primary level in Wales, universal free school meals "saves families around £500 a year per child", Ms Harwood said, but a strict means test still applied for secondary age children.
She said access to Welsh government support with the costs of school should be expanded .
"There are a lot of households living in poverty in Wales who cannot currently access free school meals or the school essentials grant," she said.
"They can't get help with the cost of school trips.
"We know there's about 25,000 secondary aged learners who are living in poverty who don't qualify for support with these grants and with free school meals."