Exam results in London have improved by nearly 40 per cent over the past six years, with some schools now doing twice as well as they were in 1991, an independent study has revealed.
It discloses improvement across the board at GCSE, with the less able pupils as well as the brighter children out-performing their counterparts of six years ago.
Researchers from Keele University's Centre for Successful Schools discovered improvements ranging from 26 per cent in Harrow to 74 per cent in Islington. The average was 39 per cent.
The study compared the GCSE results of 117 schools in 11 boroughs since 1991 in both inner and outer London authorities.
Its findings underline two of the main criticisms of official league tables - due to be published for the first time under a Labour goverment next week.
Critics say that raw statistics take no account of either past performance or the social and economic context of schools.
This year's league tables will include GCSE results from the previous three years, enabling comparisons to be made for the first time.
The study, led by Garry Gough from Keele University and commissioned by the Association of London Government, reveals that only three schools had not improved their GCSE results. It also plots the success of the 11 boroughs which took part (see table).
Schools in the bottom quarter improved their position by 63 per cent, nearly three times as much as schools in the upper quarter where improvement was 25 per cent since 1991.
Sheila Knight, ALG education chair, said: "Even with the improvements this year, the official tables can still only give very limited information and cannot hope to measure the huge amount of work put in by so many schools in the face of severe social problems.
"By comparing results over six years, our survey measures the extent of the progress made by schools in London and gives a much truer picture of how they are performing."
The analysis of GCSE results across the 11 boroughs uses an improvement index which looks at a running average of all GCSE scores. It awards seven points for an A or A*, six for a B and so on down to one point for a G.
Government performance tables provide parents with information on the percentage of pupils who gain five or more GCSE grades A to C and the percentage with grades A to G.
Mr Gough said: "The focus on five or more grades A-C can be statistically misleading. For example it fails to differentiate performance among those who gained C and above. The Government's performance tables also distort what schools do. It is increasingly the case that additional resources are put toward pupils at the C-D boundary in order to improve the number of passes and hence the school's position in the league tables."