Ministers say academies improve faster than similar schools
Academies should not be treated as a "cure-all" for England's educational problems because their performance varies widely, a report has argued.
The study on the state-funded privately run schools for the Sutton Trust education charity pointed out 72% still did not meet government benchmarks.
It also suggested rises in achievement corresponded to falls in the proportion of pupils from deprived backgrounds.
The government says academies serve a wide range of pupils.
Researchers from the Institute of Education in London drew together a range of previous research for their study for the Sutton Trust.
They highlight the variable performance of academies, a type of school which the government sees as a key weapon in turning around struggling schools around.
The report points out that of the 29 academies inspected by Ofsted, six were rated outstanding, 10 good but 13 satisfactory - a category it now suggests is no longer good enough.
And it warns that "Academies are in danger of being regarded by politicians as a panacea for a broad range of education problems.
"Given the variable performance of academies to date, conversion to an academy may not be the best route to improvement."
Ministers repeatedly point to data suggesting that academies improve at a faster rate than other similar schools in the state sector.
But the study argues that the comparison schools used in an official government report are no longer relevant to many academies.
This is because the composition of the academies' intake in terms of social class has changed over the few years that these schools have been open, the report says.
The 24 academies in a case-study featured in this report have an average of 26.7% of pupils getting the government's benchmark five good GCSEs including English and maths.
This is important because ministers have said that schools which do not reach the 30% benchmark by 2011 could be closed or turned into academies.
Overall, the proportion of children on free school meals in academies fell from 45.3% of pupils in 2003 - when the first wave of academies were founded - to 29% in 2008.
But in some schools the changes are much more dramatic than that. In one school the free school meal rate reduced from 51% in 2003 to 9% in 2005, according to the report.
The study quotes a recently published five-year evaluation of the academy programme, calling on the government to investigate whether the freedom which academies have to manage their own admission policies can serve to exclude poor children.
The Sutton Trust report also quotes research suggesting that when the changes in social class of the intake are taken into consideration, any additional gains in achievement are "cancelled out".
However, another researcher quoted in the report suggests that improvements in results are often down to changes in social mix of children at a school.
"Successful schools tend to have a 'critical mass' of more engaged broadly 'pro-school' children to start with," it says.
Schools minister Jim Knight said: "It is important to note that the number of children on free school meals (FSM) in academies has actually risen compared to the under-performing schools they replaced.
"The fall in proportion of FSM numbers is something to celebrate because it means while the number of children on FSM has gone up so has the number from other backgrounds as well, meaning a genuinely comprehensive intake.
"The report authors welcome the fact academies are attracting a wide range of pupils.
"Like them, we want poorer pupils to benefit from excellent schools on their doorsteps and that's why we are accelerating the programme to reach our target of at least 400 academies nationally.
"On average there are three applications for every academy place - a sign they are popular with parents who shunned the schools they replaced."
Chief executive of the Specialist Schools and Academies Trust Elizabeth Reid said most academies were replacing poor existing schools, and were giving young people opportunities they have not previously had from local schools in their areas.
"The fact is that academies play an important role in their local communities. As they are making a huge contribution to the revitalisation of some of the country's poorest areas, they are working closely with parents, residents, businesses and neighbouring schools to deliver a first class education."