New York City's charter schools have long been seen as the touchstone of excellence in education reform, with UK politicians regularly flying out to learn the secrets of their success.
But last week, the results of tough new state-wide tests for New York were published and they did not make pleasant reading for the much-vaunted charter schools.
Standardised tests that are aligned to the Common Core State Standards - the US attempt to introduce a national curriculum - were sat by all New York State's eight to 13-year-olds. Less than a third of students passed the tests. But while all schools did poorly, charters performed particularly badly, with some of the biggest names, such as the Knowledge Is Power Program (Kipp) schools, showing dramatic drops in pass rates compared with tests taken in previous years.
Overall, only 23 per cent of charter schools reached proficiency in English language arts, compared with 31 per cent of public schools, while the charters scored the same as public schools in mathematics. Charters outperformed public schools in the two years before the new tests were introduced.
Kipp schools have been held up by England's education secretary Michael Gove as a shining example of state-funded independent schools, achieving impressive results with challenging, inner-city students. Indeed, Mr Gove based much of his flagship free-school policy on US charter schools, particularly those operating in New York City.
But at Kipp Star College Prep Charter School, only 16 per cent of 11-year-old students were deemed to be proficient in English, and only 11 per cent of 10-year-olds reached the necessary level in mathematics.
Kipp said it recognises that the results are not good enough, but that two of its middle schools have proficiency rates that place them in the top 10 of 50 charter middle schools in the city.
Josh Zoia, superintendent of Kipp NYC, said the company welcomes the "increased rigour" of the New York State test. "Although there were some bright spots, we are not satisfied with the current results," Mr Zoia told TES. "We are fully committed to doing whatever it takes to help our students achieve these standards. We will make changes next year to both how and what we teach."
Another charter chain, Democracy Prep Public Schools, also received disappointing results at Democracy Prep Harlem Middle School, with fewer than 4 per cent of 11-year-olds passing the English test, and just under 12 per cent succeeding in the mathematics test. The chain pointed to the progress its students had made, particularly between the ages of 11 and 13, adding that they made the best progress out of all the charters in New York City.
"I recognise that many people have looked at the New York State assessment results and seen only bad," said Alice Maggin, senior director of communications. "If one only looks at proficiency I can understand why one would come to that conclusion.
"However, at Democracy Prep Public Schools growth matters most, so we are very pleased with the excellent growth exhibited by our middle school students."
But while some schools have seriously underperformed, others have achieved impressive results, such as the aptly named Success Academy Charter Schools. At one of its schools in the Bronx, 90 per cent of eight-year-olds passed the mathematics exam and 68 per cent passed the English test.
James Merriman, chief executive of the New York City Charter School Center, which helps to establish charter schools, said that the generally disappointing results are evidence that more reform is needed. "The results confirm what educators across New York City have known for some time: the majority of our students aren't on track for success in college and beyond," he said. "This is clear proof that we need continued reform of the system. We must move forwards, not backwards."