Free school meals is 'unreliable poverty measure'
Counting the proportion of pupils taking free school meals is an increasingly unreliable way of measuring poverty and the fairness of admissions policies, researchers say.
St Mary's University says free meals can be a "misleading" marker for deprivation in schools.
The study warns of the need to support the "hidden poor".
Offiicial figures recently showed two-thirds of children in poverty are now in working families.
Free school meals have been widely used as a way of looking at the social profile of schools - but the research argues that this has failed to reflect changes in the labour market and financial pressures on low-income working families.
The study by St Mary's University in south-west London looks at pupils receiving free meals in Catholic schools in England and Wales.
Faith schools have faced accusations of being socially selective - and in Catholic schools, about 12% of pupils receive free meals, compared with a state school average of about 14%.
But the study from St Mary's, a Catholic higher education institution, argues that such figures have become an "an unreliable indicator" of hardship.
It says if the same schools are assessed by another official measure of poverty - the Income Deprivation Affecting Children Index - the outcome is reversed, with disproportionately high levels of pupils in Catholic schools living in the most deprived areas.
The study says that counting poverty by the take-up of free meals can miss the "working poor" who are in temporary and often low-paid jobs, moving in and out of employment.
Figures from the Department for Work and Pensions show that children in poverty are now more likely to be in working families than non-working families.
Among children identified as being in poverty, 67% are in families with at least one parent working, the highest recorded level.
Prime Minister Theresa May has spoken of the need to help "just about managing families", where parents might be working long hours in multiple jobs.
Such working families might be facing hardship, but might not be eligible for free meals.
Eligibility for free school meals is for families in receipt of a range of benefits and with an income below £16,190 - but families on working tax credits are excluded.
Pupils taking free meals are also used to decide how much money schools receive in the pupil premium, targeting funding at disadvantaged youngsters.
The research by Prof Stephen Bullivant also highlights that the measurement of free school meals pupils is based on take-up, rather than eligibility.
There are parents who might be eligible but who do not accept free meals - because of a stigma around children being identified or because of "cultural" reasons for not wanting to accept welfare.
The research warns that focusing on free school meal eligibility can provide an inaccurate picture of the social intake of the rest of a school.
"Free school meals eligibility is taken as 'poor' children - and any not getting free meals are seen to be affluent and middle class," said Prof Bullivant.
But he says many families face "precarious lives" and are "struggling to make ends meet", but will not show up in free meals figures.
"Class inequality is a real problem in Britain affecting children's attainment. This data fails to understand different degrees of poverty," says Prof Bullivant.
"At a time when schools are facing funding struggles, a multi-faceted approach is needed to ensure that children from deprived backgrounds, who are currently unaccounted for by the system - the 'hidden poor' - receive the targeted support they need."