Pupils in some of Wales's 22 counties could have over pound;1,500 more spent on them at school than their counterparts in neighbouring areas, TES Cymru has discovered.
A widening gap between the highest and lowest-spending local authorities has been revealed in new Assembly government figures that show budgeted expenditure for schools in 2009-10.
The difference between counties is so big that one academic described the situation as "extraordinary".
The statistics reveal that in rural Ceredigion the average pupil expenditure will be pound;6,206 in 2009-10, while in the Vale of Glamorgan the figure will be pound;4,682, the lowest for the second consecutive year.
Although both authorities have increased their education spending compared with this academic year, the difference between them has grown by pound;236.
In 2006, the Wales Audit Office and a cross-party Assembly committee on school funding both expressed concern that the gap between the highest and lowest spenders had reached pound;1,000.
David Reynolds, professor of education at the University of Plymouth, said: "Since 2006, the gap has grown even larger and suggests some pupils are being disadvantaged. You can't have an education system with a postcode lottery on this scale. It is a huge concern."
The 2006 committee report recommended that the government should set minimum basic funding requirements for school staffing, accommodation and equipment, which would inform school funding decisions at national and local levels. This has not yet happened.
Chris Llewellyn, director of lifelong learning at the Welsh Local Government Association, said the situation was complicated, with differences in circumstances between local authorities affecting their spending commitments.
"The current system may be imperfect, but it's the best we have come up with," he said.
A government spokesman said it was up to councils to set budgets in line with local needs.
As they face tightening budget settlements, councils in Wales are holding back more cash than ever before, with plans to delegate just 75.7 per cent of their education budgets to schools in 2009-10 - down from 81 per cent in 2002-03. The rest of the cash - a record pound;579.3 million - will be held centrally.
Gareth Jones, secretary of the ASCL Cymru teachers' union, said: "There are considerable advantages in terms of value for money in the local autonomy of schools. Governors know best what their school needs."
Dr Philip Dixon, director of the ATL Cymru teachers' union, said: "This proves that money is not getting through to the frontlines of education. With schools facing more demands, it's disappointing that local authorities are keeping back even more money than before."
He suggested councils could save cash by pooling their central education services.
Anna Brychan, director of the NAHT Cymru heads' union, said school leaders had not seen an improved service from their authorities despite the increased restraint.
"We will advise our members to challenge these delegation rates and ask how councils can justify an ever-decreasing figure at a time when schools are expected to do so much," she said.
The government said there was no requirement on authorities to delegate a specific percentage of expenditure to schools. Rates vary based on which services are provided centrally, but the government does expect local authorities to make sure their schools are properly funded.
Mr Llewellyn said councils spent more on education than they got from the government, and where cash was held back, it was done after "extensive discussion" with schools.