Welsh secondary schools need up to pound;43 million to bring their budgets into line with the funding enjoyed by their counterparts in England, headteachers claimed this week.
Welsh secondaries receive on average pound;150-200 less per pupil than English schools, according to information gathered by the Secondary Heads'
Association Cymru from more than a third of schools.
With 215,609 secondary pupils in Wales, the shortfall works out at between pound;32.3m and pound;43.1m.
However, the per-pupil shortfall is lower than the pound;200-300 estimates the association published last December at its annual conference. They were based on a smaller sample of schools.
Jane Davidson, education and lifelong learning minister, criticised SHA Cymru for not sharing the report before publication but said she would study it closely.
She added: "The key issue is not comparison with England but whether schools in Wales have the resources they need to do the job.
"Funding in Wales is at an all-time high. Schools in Wales had their best results ever last year and the chief inspector's annual report makes it clear that education outcomes in Wales have improved every year for the past five years. That is what we and our partners in local government fund - effective outcomes."
However, the association is reiterating its call for an independent inquiry into school funding in Wales.
Brian Rowlands, SHA Cymru secretary, said: "Money goes into the system but you cannot measure its impact and tell if it was spent on the purposes intended."
The main reason for the funding gap is that Welsh schools are missing out on central grants from government, according to the SHA Cymru report. In England, schools have had direct access to initiatives such as specialist schools, excellence in cities, and leadership incentive grants that have boosted their funding by an average of pound;300 per pupil - compared to only pound;100 in Wales.
"Where we lose out is on all the initiatives that take place in England. We don't want a plethora of those but we do want the money that goes with them. We want clear evidence that the money is being passported directly to schools," said Mr Rowlands.
The SHA Cymru report was released as local education authorities began to issue budgets to schools.
Heads report a patchy financial picture for 2005-6. In the Vale of Glamorgan, schools say a 7 per cent increase has been eaten up by unavoidable costs such as performance-related pay and implementing the workload agreement.
Schools with sixth forms say post-16 funding agency Elwa is failing to fund increases in student numbers or cover inflation, while several LEAs have passed on a 1 per cent efficiency cut imposed by the Assembly government.
Anna Brychan, director of the National Association of Head Teachers Cymru, said: "We understood the efficiency savings were supposed to come from 'back-office' savings and not affect frontline services. We are very concerned as it appears to be common practice to just pass it on to school budgets."
But Chris Llewelyn, head of education at the Welsh Local Government Association, said it was unfair to expect education budgets to be protected when that would mean bigger cuts for other services. And he insisted the funding system was transparent.
"You can see what goes into the system, what the Assembly assumes is spent on education and what authorities actually spend. It's the use made of the data which is contentious.
"There is variation where some LEAs spend a little below the Assembly's indicative figures, but this year authorities as a whole spent pound;20m above what was put in for education."