US charter school provider KIPP has been forced into a robust defence of its schools after an independent study suggested their success was due to high levels of funding and less able students dropping out.
According to research, KIPP (Knowledge Is Power Programme) schools have "substantially higher" rates of attrition compared with their local school districts, with 40 per cent of African-American boys leaving between grades 6 and 8 (Years 7 and 9).
The study places question marks over the US charter school provider, which education secretary Michael Gove has drawn from in introducing free schools and academies (see box).
The report, What Makes KIPP Work, by researchers at Western Michigan University, shows that, overall, between grades 6 and 8 KIPP cohorts dropped by 30 per cent.
Taking private and public funding together, the study says that KIPP schools received an average of $6,500 (#163;3,996) more per pupil than their mainstream counterparts.
The research also claims that while KIPP enrols large numbers of ethnic-minority and low-income students, the number with disabilities and those classified as English-language learners were "greatly under-represented".
The study, which looked at 60 KIPP schools across the US, said that while they enjoy demonstrable success among the pupils who remain in the school, policy-makers should be cautious about its effects on the wider school system.
"If KIPP wishes to maintain its status as an exemplar of private management of public schools, rather than a new effort to privatise public schools, it will need to convince policy-makers and the public that it intends to recruit and serve a wider range of students and that it will be able to do so with sustainable levels of funding comparable to what other traditional public schools receive," the report says.
In a response, KIPP said it "rejected the core conclusions" of the report, claiming the findings did "not hold up to scrutiny" and that it had identified a number of "factual misrepresentations".
"Taken collectively, the Western Michigan University report suggests that the progress made by KIPP schools is fundamentally dependent on selective admission of students, high levels of attrition and private funding that supports significantly higher rates of spending than in traditional district schools. We reject these assertions."
However, Dylan Wiliam, emeritus professor at London University's Institute of Education, said the research showed that KIPP schools, while successful for hard-working children, did not improve attainment across the system as a whole.
"The Americans have learnt that it is very hard to improve an entire system," Professor Wiliam said.
"KIPP schools have provided opportunities for hard-working kids, but that seems to be just picking the low-hanging fruit."
Teaching union the NUT said instead of "tinkering with the school structures", Mr Gove should focus on improving England's provision with "well-resourced schools and properly trained and supported teachers".
The Department for Education said charter schools had a "proven track record" in some of the "poorest inner cities in America".
$6,500 - Average additional per-pupil funding in KIPP schools
The Knowledge Is Power Programme is a network of charter schools throughout the US, which provide "open enrolment" education to disadvantaged communities.
Charter schools - particularly KIPP schools - played a major part in forming education secretary Michael Gove's policies for free schools in this country.
There are 99 KIPP schools in 20 states across the US, educating 27,000 students. The schools are heavily oversubscribed and have acquired a reputation for helping young people from the most challenging backgrounds to acquire a college education.
The Western Michigan University study is not the first to accuse charter schools of selecting its students.
Teaching union the American Federation of Teachers has raised concerns over "cherry picking".