Schools would see their GSCE pass rates plummet by up to 17 percentage points if they included pupils who left before the penultimate term of Year 11, research has shown.
Education Datalab has called on the government to act on its findings and address schools' "gaming" of exam league tables.
The researchers concluded that, in a minority of cases, pupils are being "managed out" of mainstream schools before January of Year 11 – the point that determines to a large extent whether a pupil counts in a school’s results.
They also found that:
- Outcomes for all groups of pupils who leave the roll of a mainstream school early are poor, with only about 1 per cent of children who leave to go to state alternative provision or a special school achieving five good GCSEs, and 29 per cent of those who leave to go to a university technical college or studio school.
- There is a previously unidentified group of nearly 20,000 children who leave the rolls of mainstream secondary schools to go to a range of other destinations for whom outcomes are also very poor, with only 6 per cent recorded as achieving five good GCSEs.
- In some schools, the number of pupils who have been on roll, but leave at some point between Year 7 and Year 11, is more than 50 per cent of the number of pupils who complete their secondary education at the school.
- Sponsored academies tend to lose more pupils after becoming an academy. No such trend is true of converter academies.
The research was carried out in response to persistent claims that a minority of secondary schools in England use pupil moves to boost their league table results.
As part of the research, Education Datalab reweighted school league tables, to judge the impact of pupils leaving. They calculated schools' scores according to the results of all pupils who had spent some time there – not just those who were there when their final GCSEs were taken.
Each pupil's impact on a school's overall score was adjusted proportionately to the amount of time they spent there. After the weighting, GCSE pass rates for individual schools were up to 17 percentage points lower.
In total, 125 schools’ headline pass rates were 5 percentage points or more lower in the reweighted league tables.
The BBC has reported that of the 100 schools in England which would have seen the biggest drops in pass rates 62 were in London and nine were Harris Academies.
The figures also reportedly show that Harris Academy Greenwich would have seen the biggest impact on its league table position.
TES understands that in the last four years 611 pupils completed their secondary education at the school, while 333 left before the January of their final year.
A Harris Federation spokesperson told the BBC that many of its schools joined the federation because they were failing and had a high proportion of pupils considered to be disadvantaged.
"London - which is where all of our schools are located - has high pupil mobility. It is no surprise that this would be even higher in recently failing schools with very large catchment areas and in areas of disadvantage," the spokesperson added.
Education Datalab researcher Philip Nye said: "When any attempt at holding organisations or individuals accountable is as high-stakes as the school league tables system, a small minority will be tempted to try and game the results.
"Losing lower-attaining pupils represents one of the ways in which schools are still able to unfairly boost their league table position."
Last year, TES revealed that Ofsted planned to crack down on "gaming", by marking down schools that use qualifications for their own benefit rather than focusing on the needs of pupils.
The Department for Education has been contacted for comment.