Radical changes to secondary school funding based on pupils' performance in the primary years are being considered by the Assembly government.
Officials are looking at whether some local authorities should get more cash for secondaries if pupils do consistently badly at primaries.
The approach was first recommended by the Bramley review in 2007, which said the level of school resources, especially at secondary level, is vital for success.
Authorities now receive a cash pot for secondaries via the revenue support grant, based on numbers of free school meals, the amount of additional learning needs and the pupil population spread over the county.
A spokesman for the Assembly government said the changes being considered would make the grant more "outcome focused".
"Given that primary attainment impacts on secondary-level achievement, this would enable us to target resources at areas of low achievement," he added.
If approved, the changes would not directly affect school budgets, which are set by each local authority, based on their own formulas.
And it would not mean schools were funded by results, as some have feared.
But some teacher unions have still voiced concerns. Gareth Jones, director of heads' union ASCL Cymru, said: "There could be problems if the grant is based on key stage 2 results assessment. There are concerns over the reliability and accuracy of teacher assessments, which are still in their infancy."
He said there was also no guarantee local authorities would distribute the grant any differently when they received it.
Jane Hutt, education minister, revealed that officials were looking into the changes while giving evidence to a cross-party inquiry into school funding last week.
She denied claims from some members of the cross-party enterprise and learning committee that she had "shelved" the Bramley report and said she was "committed" to looking at its recommendations.
Ms Hutt also dismissed comparisons in school funding between Wales and England, despite her government's own figures revealing a spending gap of Pounds 495 per pupil.
She told the inquiry that the Assembly government had presided over "big increases" in education funding, with per-pupil spend up 81 per cent on 19992000.
"The money we have invested demonstrates we are not short-changing schools," she said.
But she admitted that "challenging" times were ahead and said councils must make the best use of funding. She added: "If we were in a period of growth I would be banging on the table to try to ensure education gets its fair share, but we are in a very difficult time in terms of budgets."
Ms Hutt also said work was under way to "rationalise" the grants system to make it less complex and bureaucratic, with officials expected to make "tangible progress" by next spring.
Teacher unions told the committee that the education "funding fog" was lifting, but there was still concern about how local authorities allocate cash to schools.
This month, TES Cymru revealed the gap in per-pupil funding between the highest and lowest spending authorities in Wales had grown to more than Pounds 1,500, while councils are also holding back more cash from schools than ever before.
Nigel Stacey, president of the ASCL Cymru and head of Dwr-y-Felin Comprehensive in Neath, said more money needed to be ring-fenced so that it reached the chalk face.
David Healey, president of the Association of Teachers and Lecturers Cymru, warned that the whole school funding system "may collapse" if wholesale changes are not made.