Under-funding, league tables, and the slower take-up of vocational qualifications in Wales could be to blame for a widening performance gap between English and Welsh GCSE students.
Welsh students matched English teenagers' GCSE performance from 1998 to 2001. But the latest comparative statistics show that English teenagers have pulled ahead again. Last summer, 54 per cent achieved five or more A* to C GCSE grades, compared to 51 per cent in Wales.
Bob Adams, research officer with the Welsh Joint Education Committee, suggested performance tables in England have led some schools to "cynically manipulate" GCSE entries, for example by targeting borderline D-grade students. He speculated that Welsh secondaries' slowness to endorse vocational qualifications, worth the equivalent of up to four good GCSEs, may also be to blame.
Wales abandoned secondary league tables and never introduced them for primary schools.
Mr Adams said: "Vocational exams have taken off slowly in Wales, and mainly for the less able.
"There are different pressures in English schools, in terms of performance tables. There is some quite cynical manipulation going on in entering pupils and targeting grade D pupils to get them over the (five A* to C) threshold."
Further analysis could determine if Welsh GCSE pass rates are lagging behind because there are fewer vocational exam entries. But differences in how the two countries collate statistics could also explain the gap, he warned.
In the Vale of Glamorgan, Barry comprehensive boys' school has seen its A*-C pass rate rise from 24 to 70 per cent in five years.
The 1,400-pupil school has introduced vocational courses in subjects such as ICT and engineering, worth multiple GCSEs.
But headteacher David Swallow, who came from a headship in Manchester, said: "There are a lot of initiatives peculiar to England and the amount of money available seems much higher - and heads can target it where needed."
Brian Rowlands, secretary of the Secondary Heads' Association Cymru, agreed schools had been slow to take up vocational courses but said: "It's all about resources. We believe the plateauing out of results at KS4 is because of the lack of resources compared with England."
The Association's own analysis of school budgets has suggested a Pounds 150-200-per-pupil funding gap.
But an Assembly government spokesperson said results continued to improve and represented real progress for Wales, adding: "We will continue to monitor the situation and work towards a made-in-Wales solution."