There has been a lack of effective monitoring of a grant designed to help some of lowest achieving children in Wales, the assembly's influential education committee has warned.
The Welsh Government needs a "firmer grip" of the Education Improvement Grant (EIG), it said.
It focused on the support for Gypsy, Roma and minority ethnic children.
Ministers said they were determined all children were "given every possible opportunity" to reach their potential.
The committee looked at the system after the Welsh Government amalgamated a series of grants designed to support groups including Gypsy, Roma, traveller and minority ethnic children into the EIG in 2015.
AMs on the committee concluded there was no evidence to show whether the amalgamation had been beneficial or not and told the Welsh Government to improve the situation.
Committee chairwoman Lynne Neagle said: "What we found was a lack of effective monitoring with the Welsh Government, regional education consortia and local authorities seemingly not working from the same page.
"This makes measuring the effectiveness of this new, streamlined grants system difficult to measure as we have no way of knowing if it is having a positive or negative impact on the children who need this extra support."
Between 2013 and 2015, only 15.5% of pupils from Gypsy and Gypsy Roma backgrounds achieved five or more GCSEs at grades A*-C including core subjects - compared to 56% of all pupils.
Figures out last month showed that had risen to 24.4%.
Ms Neagle said although this was "more encouraging," AMs were unable to pinpoint whether it was down to the new funding arrangements.
Darren Millar, Conservative education spokesman, said: "The attainment gap between some minority groups and other pupils in Welsh schools is totally unacceptable.
"The current funding model isn't working and there is a lack of accountability in the system to ensure that resources are spent effectively."
The children, young people and education committee made 14 recommendations for the Welsh Government, including:
- Keep under review the best model for funding to support the educational outcomes of Gypsy, Roma, traveller and minority ethnic learners
- Review the arrangements in place to monitor the use of the EIG and evaluate its impact, specifically in relation to these groups of learners
- Call on Estyn to undertake a review of educational provision for these learners, providing an update since its last thematic report in 2011
- Do more to improve educational outcomes amongst Gypsy, Roma and Traveller learners and minority ethnic groups with lower than average attainment
A spokesman for Education Secretary Kirsty Williams said: "While there has been an increase in the GCSE performance of the groups of pupils that this fund aims to support, we of course accept that there is still more work to do.
"The education secretary has already given a commitment that there will be a refreshed outcome framework in place for the new financial year and this remains the case."