At the bilingual Ysgol Dyffryn Conwy, in Llanrwst, pupils who want to stop learning through the medium of Welsh in Year 7 are allowed to work towards the second-language GCSE at 14 if they stay on. Those that are successful are then transferred back to the first-language exam at key stage 4.
A report published last month accused schools that moved pupils to the simpler exam of "playing the system" and said it put other candidates at a disadvantage.
In Conwy, the county with the greatest movement, just 48.5 per cent of those assessed in Welsh at KS2 stayed first language at KS3.
But Ysgol Dyffryn Conwy head Ifor Evans, who took part in the research, said the system at his school has helped to sustain the number of children studying Welsh and contributed to strong first-language results at 16.
During the past five years, an average of 95 per cent have gained a C grade or above.
He said: "We, and the parents of these pupils, believe that by age 16 a group of young people who were in danger of becoming alienated from learning Welsh have successfully been drawn back to the idea of enjoying and taking part in the promotion of Welsh culture."
According to Mr Efans, the approach is the result of a deal he struck eight years ago with parents and pupils in a bid to prevent numbers studying in Welsh dwindling. Up to 30 pupils take up the option each year at the 800-pupil school.
"Welsh performance is largely down to confidence," said Mr Efans. "After gaining these results at 14, they're willing to get on with the larger challenge of doing Welsh as a first language."
Last week's report, carried out by consultants Llais y Lli, found a "substantial" 22 per cent shift from first to second-language Welsh over a four-year period - particularly in traditional Welsh strongholds.
The team of researchers studied 72 secondary and 153 primary schools. They found that in bilingual schools, where there had been a big shift towards second-language Welsh, GCSE results in the language were 20 per cent higher than the Wales average.
Out of 24,988 pupils assessed in first-language Welsh at KS2, 19,405 went on to take first-language exams at KS3 - with more than a fifth (5,583) sitting second-language papers.