Both Tories and Lib Dems want funding to follow the pupil
Giving schools extra cash for taking poorer pupils would lead to a "modest" reduction in the attainment gap between rich and poor, a study says.
Such "pupil premium" policies will not on their own abolish the gap in attainment, according to analysis from the Institute for Fiscal Studies.
The Conservatives and the Liberal Democrats are proposing differing versions of this policy.
The government is currently reviewing its own school funding policies.
The idea of "pupil premiums" is to offer financial benefits to state schools for each individual pupil they take from a disadvantaged background.
Currently, funding is directed towards areas of disadvantaged but funds are spread out evenly across the schools in each area.
This, it is argued, means that deprivation funding is not necessarily going directly to the education of the children most in need of extra support.
Using this more targeted approach - linking funding to individual pupils - would lead to a "modest reduction in the attainment gap between rich and poor through the direct effect of extra resources", says the IFS report.
But this would depend on how the money was used by the school, says the IFS.
"This policy will not, on its own, abolish the attainment gap, which is still likely to remain large afterwards, still likely to lead to inequalities in later life outcomes and still likely to be passed down through the generations," says the report.
"In order to significantly narrow the achievement gap, interventions must be wider than changes in schools policy."
The report analysed several different variations of pupil premium policies.
The most beneficial policy would be to target extra funds in addition to existing money. This is most similar to the policy set out by the Liberal Democrats.
But the report said: "The gains in terms of extra funding for disadvantaged schools need thus to be set against the impact of the measures required to pay for them."
The Lib Dems are proposing a £2.5bn investment attached to the policy.
The Conservatives also say that their pupil premium policy would mean spending extra money - rather than re-directing existing funds.
A Conservative spokesman said: "We have always said that any pupil premium will be extra money and will not be taken from other schools. This is still our position and we'll release more details in due course."
Schools Minister Vernon Coaker said that the analysis showed that the Conservatives' proposals have "fallen apart under scrutiny".
"The Tories claim that they can increase funding for the most deprived pupils by introducing a national pupil premium, without reducing funding for any existing school as a result. This has been shown up as fantasy budgeting and a con on parents since the Tories have made clear they would cut overall funding to schools."