In recent years, London has been hailed as a shining example of educational success.The amazing rise of the capital’s schools from being the lowest of England’s nine regions at GCSE in 2003 to the best by 2011 has been cited as a model for education systems all over the world to aspire to.

But the lustre of the city’s success is about to dulled by new school accountability measures, researchers have warned.

Education Datalab, a new research organisation launched this evening, says the government’s new Progress 8 league table measure is likely to cast the achievements of London secondaries in a less favourable light.

The researchers say the performance indicator, being introduced next year, will allow schools in many currently low-ranked authorities in the Midlands and the North to improve their standing without having to actually improve their results. They will simply have to change the courses they offer.

This would hit London schools’ Progress 8 scores, which measure relative rather than absolute achievement at GCSE.

“The measure should allow the rest of England to start to catch up with London,” Education Datalab’s report states.

The main reason is that only certain subjects can count in the eight “slots” used to calculate Progress 8. The first two slots must be filled by English and maths GCSEs, the next three by other academic “EBac” GCSEs, and the last three by any other GCSE or eligible qualification.

As things stand, schools outside London are less likely to have an academic curriculum that makes the most of the new performance measures by filling all available slots. Education Datalab found that schools in Bracknell Forest, Oldham, Doncaster and the East Riding of Yorkshire have also moved further away from that position than they were a decade ago.

However, to boost their rankings under the new system, all they have do is enter their pupils for the correct subjects - and because Progress 8 measures progress rather than absolute attainment, any grade at all will improve their position.

London schools are much more likely to have already filled all their slots. And because Progress 8 is a relative measure, they will also suffer comparatively in the rankings as schools in other areas of the country improve by changing their curriculums.

The researchers warn that this could have unintended consequences. “Schools already offering a traditional curriculum can only improve by raising individual subject grades,” their report says.

“This presents a rather unanticipated risk: schools with a traditional curriculum - those with higher-entry attainment of pupils, grammar schools, schools in London - focus their energy on improving teaching and grades since this is the only way to raise Attainment 8 [students’ average grade across these eight subjects].

“Meanwhile, the reorientation of the curriculum distracts others from maximising individual subject grades.”

Professor Simon Burgess helped to devise Progress 8 but has already warned that it could incentivise schools to “ignore the low-performers”.

Responding to the Education Datalab report, the Bristol University academic said: “Measurement is rarely neutral. People and organisations respond to being measured. Sometimes, if policy-makers have done a good job, they respond in the way intended. But people are smart and will respond to the fine detail of whatever is measured, rather than to the spirit.”

Related stories:

Row breaks out over cause of London’s state school success - February 23, 2015

Progress 8 may not mean an end to gaming the system - February 23, 2015

Success of London schools explained exclusively by pupil ethnicity, research finds - 12 November, 2014

Tristram Hunt: Labour will bring back the London Challenge - 21 September, 2014

The London wonderground - 27 June, 2014

Primary schools behind boost in London GCSE results - 23 June, 2014