A teaching union claims a gap in funding is part of the reason for a small drop in A-level results in Wales.
But ministers say education spending has never been higher, and expected the union to be "more positive" about the achievements of pupils and staff.
The overall A-level pass rate dipped a little to 97.1%, while in England and Northern Ireland it is 0.5% higher.
The National Union of Teachers Cymru says the dip in results is "not a disaster," but is a concern.
The gap between A-level performance in Wales and the rest of the UK widened for the second successive year when results were announced on Thursday.
Across the UK the overall pass rate rose to 97.6%.
David Evans, Welsh secretary of NUT Cymru, said the drop in Wales' A level pass rates "may only be minor at this stage, [but] we don't want to see it increasing year on year and we're back here in three of four years' time with some disastrous results."
He told BBC Radio Wales: "It's not a disaster now. It is a concern. We're not saying that the reason for it is solely because of the funding but it is a reason."
He said the £527 funding gap between students in Wales and England was having a detrimental effect on pupils' performances.
Mr Evans said the union welcomed a commitment made earlier this year to close that gap
"We pushed for the closing of the funding gap bug the commitment is in fact one per cent above the block grant that's given to Wales,
"And all the indications are that its going down, its one per cent effectively of nothing. And one per cent a year over the next few years isn't going to close the five, six and seven per cent gap that currently exists between Wales and England.
Mr Evans said the union also welcomed a funding review announced by the assembly government,
But he added that the money which is going to front line teachers and into the classrooms was still not enough.
Gareth Jones, chair of the assembly's enterprise and learning committee, said there was no evidence that funding was behind the dip in A level results.
He said further analysis and a look into how social and economic policies, including child poverty, was needed.
An assembly government spokesperson said: "We would have expected the NUT to be more positive about the achievements of Wales' children and young people - and their teachers - especially those who have succeeded in gaining additional points for entry to university by following the Welsh Bacc.
"Successful candidates have, by completing the core of the Bacc, achieved the equivalent UCAS points awarded for an 'A' grade at A' level.
The spokesperson added: "School funding in Wales has increased by 71% since 1999-2000. We have made it clear that we want to ensure more funding reaches the education front line - schools, colleges and universities and that's why we commissioned Price WaterhouseCoopers to review the cost of administering education in Wales.
"We've also made a commitment to increase funding by 1% more than changes in the Barnett formula."