School spending per pupil in Wales will have fallen by almost a tenth over 10 years, according to new research.
One economist estimates there will have been a cut of 9% or £500 per pupil in real terms between 2009-10 and 2020-21, if spending plans stay the same.
Ministers said despite "considerable financial challenges" spending has not fallen at the same rate as in England.
The Welsh Government has previously committed to increasing spending on school standards by £100m in this Assembly term.
Economist Luke Sibieta, a research fellow at the Institute of Fiscal Studies and also the Education Policy Institute, said this would fall short of what was required to avoid a real terms cut.
- Spending per head fell 5% up to 2017-18 - this is a smaller cut than seen in England and Northern Ireland but bigger than in Scotland
- This was mainly due to pupil numbers remaining stable in Wales and Scotland while they increased in the England and Northern Ireland
- Total day-to-day spending on schooling in Wales was just over £2.5bn in 2017-18
- Total spending per pupil was about £5,790.
Mr Sibieta's paper, submitted to an Assembly inquiry on school spending, said that would take spending back to a level last seen in the mid-2000s.
The paper also looked at variations in spending across local authorities, from about £5,000 per pupil in the Vale of Glamorgan to £6,400 per pupil in Ceredigion.
'Everybody's slightly passing the buck'
The Welsh Government gives funding to councils to spend on local services, including education.
When the Welsh Government calculates how much each council receives, it allocates an amount for education based on factors including deprivation and the distance pupils will have to travel to go to school.
However, when councils receive their settlement, it is up to them how they divide that up between services and what they spend on schools.
It is a system that has left parents at Crickhowell High frustrated. While Powys's education budget is relatively high compared with many other areas, the Level the Playing field campaign group claims Crickhowell High is one of the worst-funded schools in Wales.
They said funding has become so tight teachers are buying basic school supplies and classrooms are overcrowded.
Parent Eliane Wigzell said: "There's a tendency for [councillors to say] 'that's their responsibility, Welsh Government set out education funding', or Welsh Government say county councils delegate the funding, and everybody's slightly passing the buck."
Another parent Marie Bennett said: "All children should have the equal amount of funding, regardless of where they live, especially looking to the future with the new curriculum coming in".
Sarah Pritchard, also a member of the parents' campaign group, added that she had been "completely blind" to the issue until it was explained, because the school was so successful.
Powys Council said claims Crickhowell High School is treated unfairly were untrue.
"Powys remains committed to funding its schools appropriately, providing an extra £1m in the 2019-20 budget," said a spokeswoman.
"The county is the largest overall funder of schools per pupil in Wales but recognises the challenges they face in difficult financial times and remains committed to working with all schools to provide the best possible learning experience."
The Welsh Government said the UK government's "sustained austerity agenda" had led to substantial cuts to Wales' overall budget, while it was also waiting for the outcome of its comprehensive spending review.
"However, in spite of these considerable financial challenges, and as acknowledged by the report, funding per pupil has not fallen at the same rate in Wales as it has in England and Northern Ireland," said a spokesperson.
"We recognise that to continue to raise standards, our schools and teachers need additional support.
"That is why we recently announced the single biggest investment for teachers since devolution - a £24m package of professional learning to support the new curriculum, giving schools the time and resources they need."
A UK Government spokeswoman said the overall block grant sent to the Welsh Government means funding per person remains at least 15% above the comparable spend in England.
"It is for the Welsh Government and Welsh local authorities to determine the nature of that spend," she said.