They obscure more than they reveal, according to the most detailed analysis of data behind the tables yet carried out.
Our study of the results of 135 secondaries suggests the Government's rankings of the most rapidly improving schools is skewed towards recognising those which have recently introduced vocational courses.
Intermediate general national vocational qualifications are worth four GCSEs in the tables.
And the schools at the top of last year's government list of the most improved in England predominantly have pupils taking GNVQs in more than one subject.
Schools at the top of the most-improved list tend to obtain about three GCSEs for league-table purposes from GNVQs. For those in a control group of 47 secondaries, the average is about half a GCSE.
There is also some evidence to suggest that pupils find vocational courses much easier. Among the top 30 most improved schools, the average pass rate at C or better for GNVQ ICT was 83 per cent, compared to 39 per cent for GCSE maths.
Our research also confirms that the top most improved schools would fare much worse under a new government measure used to rank secondaries: the proportion of pupils gaining five or more top passes in subjects including English and maths.
Perhaps the most remarkable finding, however, relates to what subjects pupils are being offered in the most-improved schools.
On average, a pupil attending a most improved school is less likely to achieve a GCSE at grade C or above for academic science, or a European language than at a secondary in our control group. Many of the 100 most improved schools also had low numbers of pupils passing history and geography GCSE.
This finding is potentially devastating, as it suggests that league tables are an inadequate guide to schools' performances in subjects that many parents regard as of central educational importance.
The TES sought, under the Freedom of Information Act, detailed GCSEGNVQ results for 2005 from the Government's list of the 104 most improved schools in 2001-4, plus a control group of 60 schools. Responses were received from 135 secondaries overall. A paper providing a longer analysis of the results is available at www.tes.co.uk or by email: firstname.lastname@example.org
Roger Titcombe and Roger Davies