It now costs around GBP2,275 a year to send a child to a secondary state school in the UK, according to a report from charity Child Poverty Action Group (CPAG) and the Centre for Research in Social Policy (CRSP).
The cost of sending children to state school has increased by GBP520 in the last three years, a report has found.
The cost for primary school has also increased by 16% and now exceeds GBP1,000 a year.
Similar research in 2022 set the annual cost for a secondary school child at GBP1,755 and nearly GBP865 for sending a child to primary school.
The "significant" increase since 2022 outstripped both inflation and earnings growth during this period, the research suggests.
Some of the key drivers behind the rise are higher costs of food for packed lunches and snacks for the school day as well as an increased need for access to technology for digital learning.
Additionally, the research found a higher cost associated with secondary school pupils having to participate in and supply materials and equipment for subjects like design, on top of costs like textbooks and stationery.
CPAG has called on the government to use its upcoming child poverty strategy to improve the living standards for families.
The charity suggests this could be achieved by helping with the cost of the school day, including an expansion of free school meals.
"Parents are struggling to cover household bills while also forking out for pencils and PE gear at school. And still their children get priced out of school activities," Kate Anstey, head of education policy at CPAG, said.
"Help with the cost of the school day - including an expansion of free school meals and cash support with uniform costs in England - would make a huge difference to parents and kids alike."
Ms Anstey said that unless the government's strategy "scraps the two-child limit, more and more children across the UK will see their potential - in and outside the school gates - stunted by poverty".