School budgets have fallen by around £370 per pupil in real terms in six years, research by BBC Wales has found.
The Welsh Government has had its budget cut by UK ministers in that time, but has promised to protect schools.
One primary school head teacher said he has cut a quarter of his staff while teaching unions says the cuts "seriously undermine" schools.
The Welsh Government said it is putting money into raising standards and targeting cash at poorer pupils.
The 6.2% cut, estimated by the Wales Live programme, takes into account the impact of inflation between 2010-11 and 2016-17.
Since he became first minister in 2009, Carwyn Jones has described school funding as a priority.
Budgets increased in cash terms in five of the six years between 2010-11 and 2016-17, but the gains are wiped out once inflation is taken into account.
The per-pupil funding figure takes into account an individual school's budget, as well as money spent centrally on schools by each local authority.
It also includes additional funding from the Welsh Government, such as the Pupil Development Grant which is targeted at poorer pupils.
The Welsh Government has said comparisons between the amount of funding per pupil in Wales and England are no longer viable because of changes to the way schools in England are run.
But one teaching union, the NASUWT, estimates the gap between the two countries is now £678 per pupil in England's favour.
According to the Institute for Fiscal Studies, day-to-day spending per pupil in England was "largely frozen" in real terms between 2010-11 and 2015-16.
Doing the sums
On average, £5,418 was budgeted to be spent per pupil in 2010-11.
In 2016-17, the figure was £5,570 - an increase in cash terms.
But we used Treasury data to adjust for inflation and work out what the 2010-11 amount would be worth in 2016-17 prices.
The answer is £5,959.
We also then slightly inflated the 2016-17 figure, to reflect the fact that some money for the Flying Start scheme was taken out of schools budgets in 2015-16.
That gave us a revised 2016-17 figure of £5,587, £372 lower in real terms than the 2010-11 amount - a cut of 6.2%.
Steve Rees is the head teacher of Evenlode Primary in Penarth, near Cardiff, and a representative for the National Association of Headteachers.
He told the programme cuts meant his school was starting to look run down, and it was also affecting pupils.
"It's had a dramatic effect," he said.
"It's a quarter of the staff that's disappeared in the last four years and that's basically down to the fact we can't afford to employ them.
"Obviously if you've got targeted children that benefit from additional support those children simply aren't getting it any more.
"It means more able children that could do with being pushed a bit further we're unable to support them in the way we have been able to."
His school has a low proportion of children eligible for free school meals, so doesn't qualify for much of the Welsh Government's Pupil Development Grant.
He felt big variations between schools were unfair. "It makes me feel very frustrated," he said.
"It makes me feel very angry. I've got great kids but they're simply not getting a fair deal."
"Welsh Government needs to sit back and look at priorities because at the moment from my perspective there are too many initiatives that I simply cannot cover with the amount of money I'm given."
Education Secretary Kirsty Williams told Wales Live public finances are "challenging" and "unfortunately education has not escaped the consequences" of falling Welsh Government budgets .
"This year we will spend £93m on Wales' poorest children," she said.
"I've been very clear about prioritising closing the attainment gap in Welsh schools and I'm not going to make any apologies for targeting that money at those children's education that need it the most."
She said £20m would be spent on children with additional learning needs, adding they were "again a category of children who have not perhaps in the past got the support they need".
"I would love to spend more money on Welsh schools, and that's why I've taken money out of my education budget, I've put it into the revenue support grant to local government so that they can support the front line," she added.
A Welsh Government spokesman said: "Raising standards for all and tackling the attainment gap is at the heart of our new national mission for education.
"That's why we are supporting teachers and learners by investing £100m to raise school standards over this Assembly term, maintaining our support for delivery of the Foundation Phase, and providing more than £187m over the next two years through our Pupil Development Grant - helping to break the link between poverty and attainment."
He added there had been improved GCSE outcomes and a narrowing of the gap between poorer students and the better-off.