Welsh Assembly officials were this week accused of misleading Assembly members over comparisons between school funding in Wales and England.
The Assembly's education committee was told last week that comparisons of how much is spent directly by schools and on school services is no longer possible because of changes in data collection in the two countries.
Officials said a new database, due in Wales in 2006-7, would not help as it does not record income received by schools. Instead, committee members were given data that compares local education authorities' total per-pupil spending on education - including spending on adult and youth education and LEA administration.
The consistent financial reporting (CFR) database, already in use in England, records school income in detail, down to insurance cover for sick teachers and council rebates.
The Secondary Heads Association Cymru, which has campaigned for an independent inquiry into school funding, said officials had misled Assembly members over the CFR.
Its research, published last month, suggested Welsh schools were pound;150-200 per-pupil worse off than similar schools in England. The association believes Welsh schools are underfunded, but says sensible debate about the situation is impossible because of the "funding fog" over the figures.
Brian Rowlands, the association's secretary, said: "The CFR gives you each school and its income from delegated budget and special grants as well as school-generated income. It also gives expenditure, capital funding and forward balances, so comparisons can be made.
"Inclusion of non-school education for youth and adults is another confusion. They still have arguments about funding in England, but at least you have the statistics."
Opposition parties this week tried to force a debate on school funding, and there were calls from the Tories for education and lifelong learning minister Jane Davidson to stand down.
Mark Isherwood, Tory co-education spokesman, said she had moved the goal-posts by including adult and youth spending in the comparative figures given to the education committee.
"The minister has misled the committee and she should consider her position," she said. "I condemn the way she continues to hide behind a smokescreen of figures without acknowledging the serious concerns raised by SHA Cymru. We all know the education budget has gone up significantly. But head are asking, 'If that's the case, why are we having to make cuts?' We have to listen to them because they have no political axe to grind."
Janet Ryder, shadow education minister, also accused the Assembly government of misleading members over the CFR.
"People are always going to ask for more money but the first step is to be clear about what's going into schools. We get announcements of huge funding from the Assembly, then it works through LEAs to schools and we can't track it.
"There is more money going into education but there are pressures in schools to respond to new initiatives, like workload reforms, which are increasing the amounts going out of their budgets."
An Assembly government spokeswoman denied the committee had been misled by the minister or officials, and said the CFR database could not be used for comparative analysis between school funding levels in England and Wales.
She said: "The report to the committee was about budgets set by LEAs and comparisons with England.
"CFR cannot provide that information. CFR analyses income and expenditure for individual schools and can enable comparison to be made between individual schools.
"In due course, that will be possible for schools in Wales but it will still not replace analysis of budgets set by authorities for their schools and for school services.
"Comparative data with England is becoming harder to verify accurately.
Officials therefore made it clear in their presentation that the paper presented to committee is the only basis on which a comparison can be made with the position in England as there are differences in England and Wales in the arrangements for reporting school budgets and individual school's budgets.
"Neither the Minister nor officials misled the committee. On the contrary, the Assembly government is seeking to clarify information in the interests of accuracy and intent."
Figures presented to the education committee on LEAs' total per-pupil expenditure show that the gap between England and Wales is closing. This year it stands at pound;157. But if London is excluded, the gap shortens to Pounds 23.