More must be done to ensure that government funding for poor pupils is helping these youngsters to succeed at school, according to a group of MPs.
A new report by the Commons Public Accounts Committee (PAC) argues that there is some evidence that the achievement gap between rich and poor youngsters has started to narrow since the introduction of the Pupil Premium in 2011.
But it warns that there are still "inequalities" in funding, and suggests that the government needs to do more to support schools so they can share ideas and use the funding effectively.
The Pupil Premium is funding given to state schools in England to help improve the results of disadvantaged youngsters - those eligible for free school meals.
This year it is worth £1,320 for primary school children and £935 for secondary school students.
Around two million youngsters in England come from disadvantaged backgrounds, the committee noted.
In the last four years, headteachers have started to pay more attention to closing the achievement gap and there are examples of schools using the money on schemes that are successful in boosting results, the committee's report said.
But it goes on to say that more work is needed to help weak schools to learn from those that are using the Pupil Premium effectively.
The committee also called for the government to set out a clear timetable to review school funding, claiming that under the current system, some schools received around £3,000 a year more than others for each poor pupil.
The report said: "The Department for Education needs to be better at supporting schools to share and use best practice more consistently so that more schools use the Pupil Premium effectively.
"In addition, there remain inequalities in the core funding received by schools with very similar levels of disadvantage.
"As the impact of the Pupil Premium will take a long time to be fully realised, the Department needs to do more to demonstrate its emerging benefits in the meantime."
PAC chair and Labour MP Meg Hillier said: "Figures show there has been some narrowing of the attainment gap but results have been uneven.
"More joined-up thinking is required.
"It is clearly in children's best interests that weaker schools learn from successes elsewhere and new measures are needed to ensure this happens."
She added: "There needs to be a better understanding of why disadvantaged pupils from different backgrounds can perform so differently; similarly, the department should clarify the options open to schools where parental disengagement is seen as an obstacle to improvement.
"The committee is very concerned the roll-out of Universal Credit will make it harder to identify children eligible to benefit from the Pupil Premium - and indeed, the department admits it still has no plan in place to address this.
"We are therefore asking for an update on its progress within six months.
"We also want to see a clear timetable for the department to review the schools' funding formula which, at present, sees some schools receive about £3,000 a year more than others per disadvantaged pupil."
Sir Kevan Collins, chief executive of the Education Endowment Foundation (EEF), said: "There is no doubt that the Pupil Premium has enabled schools to do more to improve the outcomes of their less advantaged pupils.
"But equally, the committee is right that we still have much to do to ensure that all those from poorer families reach their potential, particularly those in coastal and rural communities.
"As we learn more about what works, it is vital that after the spending review, the Pupil Premium continues to be paid to schools for all their disadvantaged pupils, regardless of their attainment, so that they all achieve their potential."
A Department for Education spokesman said: "It is encouraging that this report recognises the extent to which the Government has been able to narrow the attainment gap between disadvantaged pupils and their peers.
"We are determined to ensure every child, regardless of background, is given an education which allows them to realise their full potential and we are committed to do more to tackle educational inequality.
"We will now consider the findings of the Public Accounts Committee carefully and respond later this year."
Shadow education secretary Lucy Powell said: "We have been warning the Government for some time that the introduction of Universal Credit will have knock-on consequences for allocating the pupil premium. It beggars belief that they still haven't resolved this issue.
"Ministers obviously still have their heads in the sand. With the attainment gap between poorer pupils and their peers widening, it's vital that the Government takes urgent action to ensure that disadvantaged pupils don't lose out and that the pupil premium is effective."