Schools in poorer authorities are to lose hundreds of thousands of pounds as a result of changes to post-16 funding, while richer authorities will see their budgets increased.
The new system will leave the poorest three authorities, Merthyr Tydfil, Caerphilly and Blaenau Gwent, collectively pound;1.3 million worse off.
But the Vale of Glamorgan, traditionally one of the most prosperous areas in Wales, is the biggest winner of the new system, gaining an additional pound;1.38 million. The changes follow the announcement this week, by Education and Learning Wales, the organisation responsible for financing post-16 education, of its new funding system. This will be implemented from August 2005.
Under the new system, both sixth-forms and further education colleges will be provided with a fixed unit of funding for each course offered. The aim of the new system is to even out sixth-form funding, regardless of location or type of institution.
Additional funding will be available for extra costs, such as transport in rural areas, or Welsh-medium provision.
ELWa has produced a guidance table for sixth-form heads, demonstrating how the funding changes would affect this year's budget. Under these projected changes, Merthyr Tydfil would be pound;643,643 worse off, while neighbouring Caerphilly would receive pound;492,876 less. Blaenau Gwent's budget would be reduced by pound;248,793.
These three authorities are among the poorest in Europe.
Meanwhile, Cardiff, a relatively prosperous authority, would gain an additional pound;646,692 in funding. Rural Powys is the greatest loser, with a budget cut of pound;646,976.
Anna Brychan, director of the National Association of Head Teachers Cymru, said: "We are mystified how, if rurality and social deprivation are taken into account, the poorer authorities are still the ones who lose out.
"A situation where richer communities gain at the expense of the poorer is not, we hope, the political agenda that Wales wants to pursue."
ELWa, currently an independent quango, will be brought under direct control of the Welsh Assembly government from April 2006, it was announced last month. Ms Brychan believes that the new funding system runs contrary to the Assembly's policies.
She said: "This makes a mockery of the Welsh Assembly's commitment to equality of opportunity and social inclusion.
"If the amount of money to schools is reduced by this amount, it could also lead to redundancies. In a school, you can't confine that to post-16.
Taking money away could have a destabilising effect on the entire school community."
An ELWa spokeswoman said: "The new system has been designed to remove the 'postcode lottery' in Wales, where different parts of the country receive significantly different levels of funding for the same learning experiences.
"These historical funding practices have led to significant funding disparities, which are neither sustainable nor justifiable. The new arrangements promise greater fairness and transparency, that will help to clear the funding fog."
At the moment, spending for education is not allocated directly to schools or colleges by the Welsh Assembly. Instead, each local authority is awarded a lump sum, which it divides up according to its own priorities. Spending therefore varies from authority to authority.
Secondaries in Ceredigion, for example, receive almost pound;700 more per pupil than those in Denbighshire. Brian Rowlands, secretary of the Secondary Heads Association Cymru, welcomes any attempt to tackle the inequalities created by this system.
"We're pleased to see the end of the postcode lottery," he said.
"Inevitably, there will be winners and losers, and we trust that transitional arrangements will ease this. Having recognised the principle of post-16 equality, why not recognise pre-16 equality?"
A spokesperson for the Assembly said: "The allocation of resources for pre-16 education is a matter for local government. We have always encouraged LEAs to spend well on education."