Even relatively well-funded schools in Wales are getting less cash than comparable schools in England, according to a report compiled for the Secondary Heads' Association Cymru.
The difference could add up to pound;120,000 a year for a 600-pupil school, says the SHA, which is seeking a Daugherty-style inquiry into funding.
Meanwhile, a senior local government figure has warned that education budgets could bear the brunt of any cuts in council spending. And the Assembly government has delayed publication of key statistics on how much it thinks councils should be spending on education.
John Hopkins, SHA Cymru's outgoing president, said the report's findings were so worrying that the association plans to collect data from more Welsh schools.
"For some time, we have been picking up anecdotal evidence that English schools appear to be better funded than we are, because there is direct money going into them - for example, for specialist status," he said.
"These figures show that even better-funded schools in Wales seem to be getting pound;2-300 per pupil less than the average for schools in England in lower-funded areas. That adds up to a pound;120,000 difference for a 600-pupil secondary.
"Since many of the Welsh examples are in Powys they are among the better funded in Wales, so we expect the gap to widen as more schools are added to the model."
The funding gap was of particular concern given demands on heads to deliver workforce reforms and increase curriculum choices for 14 to 19-year-olds, when councils were warning of standstill or even reduced budgets, he added.
"We are calling on Jane Davidson, education and lifelong learning minister, for a review. She says she is evidence-based and we have evidence of a problem."
The SHA Cymru report, which was presented to the association's annual conference in Llandrindod Wells yesterday, compares 17 Welsh secondaries with similar-sized schools in England. They are drawn from lower-funded local authorities in the south-west and north-west of England.
But an Assembly spokeswoman said: "The latest figures (2003-4) show that local authorities in Wales spent pound;26 per head more than those in England on schools. This year the average per-pupil spend in Wales has increased to Pounds 4,039 from pound;3,668 and the range between the highest and lowest-spending authorities has fallen.
"Local authority budgets for education have increased this year by 5.8 per cent in Wales. In England, the planned increase in local authority spending on education was 5.5 per cent."
Meanwhile, John Davies, the education and lifelong learning spokesman for the Welsh Local Government Association, has warned that education budgets could be hardest hit by this year's local government settlement.
Speaking at the WLGA's council meeting last week, he also told first minister Rhodri Morgan that there was not enough cash for workforce reforms. The Assembly government has also come under fire for delaying publication of its assessments of how much councils should spend on education.
Ms Davidson told members of the National Union of Teachers Cymru last month that early publication of the figures would "help inform dialogue" about council spending decisions in local school budget forums.
But the figures may not now be available until January. Sue Essex, the Assembly's local government minister, wants more time to consider the responses from the WLGA and others to the Assembly's draft budget. Gethin Lewis, secretary of NUT Cymru, said the delay was disappointing.
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