Thousands of pupils at England's most improved secondary schools are being steered away from exams in academic science, languages, history or geography, The TES can reveal today.
The findings may have implications in Wales, where some schools are making more use of vocational qualifications to motivate pupils at risk of disaffection.
Barry boys comprehensive, in the Vale of Glamorgan, saw results shoot up from 53 to 70 per cent of pupils achieving five or more A*-C grade GCSEs in 2004, and was named Wales's most improved school last year. Results dropped back to 61 per cent in 2005 but are still significantly above its 2000 figure of 37.
Head David Swallow attributed the school's success in part to a more vocational curriculum that suited the needs of pupils.
The TES survey adds fuel to the debate about whether vocational qualifications are equivalent in standard to GCSEs.
The Westminster Government has argued that General National Vocational Qualifications (GNVQs) should be worth four GCSEs because schools devote much more time to teaching them. But our analysis reveals that the top 30 most improved schools spent 3.6 hours a week teaching GNVQ ICT, compared to 3.3 hours for maths GCSE.
Average GNVQ pass rates in ICT were 83 per cent among the top 30 most improved schools in England, compared with 39 per cent for maths GCSE.
Teachers were this week divided about some schools encouraging pupils to take vocational rather than academic courses. Some believe the former are motivating. Others argue that pupils are being denied the opportunities offered by academic qualifications.
In England, the TES survey also raises questions about the extent to which inner-city schools are abandoning academic study to boost their league table places.
At five of the seven schools named by ministers as England's most improved last year, not one pupil passed double science GCSE at C grade or better.
National GCSE league tables are published next week in England, but were scrapped in Wales in 2001.
The TES analysis suggests that the tables, in which popular vocational courses are counted as four GCSEs may be encouraging, schools to move away from more academic provision.
Nine of the 100 most improved schools had no pupil gaining a C in history, while the same was true in some schools for geography and languages.
At the North school, a secondary modern in Kent named as England's equal most improved last year, no pupils achieved a C or better in double GCSE science in 2005. Some 12 per cent did so in a language, 2 per cent in history and 8 per cent in geography.
Professor Alan Smithers, of Buckingham university, said GNVQs are not valued by employers and so are of little use.
He said: "The lives of young people with talent in subjects like physics or history will be blighted if they end up in schools where they cannot study them."
But Tony Broady, head of Walker College, in Newcastle, said: "Introducing courses that pupils can achieve in is not something to be ashamed about.
It's something to be proud of."
Of the top seven listed schools, the average figure for the percentage of pupils achieving a C or better in double science GCSE was 6 per cent; in at least one language, 6 per cent; in history, 6 per cent; and in geography, 5 per cent.
Ministers want all pupils to be given a choice of vocational or academic courses. But the numbers opting out of subjects many parents would regard as central suggest they are becoming options for a small minority in some comprehensives.
The TES combined with Roger Titcombe, a retired headteacher, and Roger Davies`, a statistician, to compile the data.