Schools with high levels of pupil poverty and low attainment need extra cash - not a "pull up your socks" attitude.
The verdict follows research into school funding in Wales by Glen Bramley, professor of urban studies at Heriot-Watt University in Edinburgh. He said the government must set a higher premium for poverty-stricken schools to help break the cycle of low achievement.
"The rhetoric from some politicians is that it's just about schools pulling their socks up, but home background is the most important factor in attainment," he said.
Two sets of figures released by the government last week show that affluence and ethnicity are still big factors in pupils' achievement.
One report, Academic Achievement and Entitlement to Free School Meals, shows that children eligible for free schools meals (FSMs) - 17 per cent in Wales - performed below average at each key stage.
It also shows pupils in three local authorities - Denbighshire, Blaenau Gwent and Torfaen - underperformed at KS3 and GCSE, considering their socio-economic background.
Last month, TES Cymru also found only a minority of primaries with a high proportion of FSM pupils bucked the trend with glowing Estyn reports. Most schools with above-average poverty still struggle to hit targets for core subject indicators and GCSE passes.
Kirsty Williams, Lib Dem education spokesperson, said the difference in schools' achievement was "social injustice at its worst".
"We not only need to fund pupils from disadvantaged backgrounds properly through a pupil premium, but we need to strengthen the links between home and school, recognising that children's early development depends critically on the relationship between school and family," she said. "There is also inconsistency as not all eligible pupils register for free meals."
Professor Bramley has called for an overhaul of the current funding formula, and for factors associated with poverty to be taken into account.
Schools with FSM entitlement above 20.1 per cent are eligible for RAISE funding, but they receive little extra in relation to levels of pupil poverty.
"To overcome their deprived backgrounds and get a reasonable proportion of pupils achieving, more resources are needed," Professor Bramley said.
He added extra school spending has a small effect on attainment, particularly at secondary level.
A second government report, Academic Achievement by Pupil Characteristics, also showed wide ethnic variation. The gap between the highest-performing ethnic group (Chinese) and the lowest (Black) is consistent throughout primary and secondary; at KS3 and 4 the difference is more than 30 per cent.
The report also found that only one in 10 pupils with special needs reached the expected standards in the core subjects by the age of 15, but for pupils with statements the figure is only 6 per cent.
Professor Bramley said there was evidence that high spending on special needs pupils also had a positive impact on achievement.