Welsh-medium secondary schools are getting better GCSE results overall than their English-medium counterparts - but they are also catering for fewer pupils from disadvantaged backgrounds.
However, those Welsh-language schools serving disadvantaged areas may be struggling to maintain improvement rates, according to an academic.
Assembly government statistics show that this year's pass rate for five A*-C grade GCSEs or equivalent ranged from 14 to 90 per cent among Wales's 220-odd secondary schools. Free school meal (FSM) entitlement ranged from 2.3 to 64 per cent, with a national average of 16.6.
But in the Welsh-medium sector - which makes up a quarter of all secondaries - the highest level of FSM entitlement was 19.7 per cent, and most schools had lower FSMs than five years ago. GCSE results for five good passes ranged from 41 to 87 per cent, with only four schools failing to achieve the Welsh average of 52 per cent.
In the English-medium sector, nearly four out of 10 secondaries (62) had FSMs of more than 20 per cent in 2004-5. Of these, only four achieved a 52 per cent pass rate.
Unions put the success of Welsh-medium schools down to the commitment of their staff, parents and pupils.
Gruff Hughes, general secretary of UCAC, the Welsh-medium teachers' union, said: "Staff are dedicated to Welsh-medium education and want to make sure it succeeds. I'm not denigrating other schools, but Welsh-medium staff do lead extra work outside the classroom and encourage children who perhaps don't have the language at home."
Geraint Davies, secretary of teaching union the NASUWT Cymru, said it would be wrong to assume Welsh-medium schools were situated in better-off areas.
He added: "Parents choose to send their children to particular schools, and they have to go out of their way in some parts of Wales for a Welsh-medium education."
But David Reynolds, professor of education at Plymouth university, who lives in Wales, said the statistics suggest Welsh-medium schools in more deprived areas may be struggling to improve their results further.
"That's surprising, because they were the big successes when the sector got going," he said.