Specialist schools are outperforming other secondary schools in England, the education watchdog Ofsted has said.
Some specialist areas seem to have benefited more than others
Ofsted chief David Bell says teaching is better in specialist schools and they have a greater "sense of purpose".
But, while GCSE-level results overall were improving, grades in schools' specialist subjects were often too low.
Specialists, which receive extra funding to be centres of excellence in particular subjects, are now in the majority among secondary schools.
Mr Bell's report also says that other schools nearby did not seem to suffer because of specialists' success.
In a generally positive report on the impact of specialist status, Mr Bell, chief inspector of schools in England, said the schools were showing "significant improvements".
Almost two-thirds of secondary schools in England are now specialists, in which schools become a centre for excellence in a particular subject area, such as technology, the arts, sport or languages.
Where there is a weakness, the report says it can be in how specialist schools teach their own specialist subjects. Results can be lacklustre. Also specialist schools can lack support for the most able pupils.
But overall the report strongly endorses the achievements of the specialist school programme, which in a few years has become the most common type of school for secondary pupils in England - there are none elswhere in the UK.
"Being a specialist school makes a difference. Working to declared targets, dynamic leadership, a renewed sense of purpose, targeted use of funding and being a contributor to an optimistic network of like-minded schools, all contribute to a climate for improvement and drive forward change," said Mr Bell.
Miriam Rosen, Ofsted's director of education, said: "Pupils in specialist schools are performing better at GCSE and results are improving faster than in other schools.
"However, we have also found that the rate of improvement in the specialist subjects has levelled off in the last three years and in some subjects has declined," she told BBC Radio 4's Today programme.
"One of the things it tells us is that when you are doing well it is actually very difficult do even better and it's harder to improve results the higher the starting point is.
"But it also tells us it is necessary to look in detail at the individual specialist subjects and to look at reasons why any one of those subjects might not be doing as well as others."
Better and worse
Specialist status is no guarantee of improvement and the average performance masks some wide variations.
Figures produced by the Specialist Schools Trust, relating to the 1,360 specialist schools with results from last summer's exams, show 66% had better performance than they had had four years earlier.
In 55 schools (4%) the results were the same. In 401 (29%) results were worse than they had been in 2001.
For example, at Hartsdown Technology College in Kent, GCSE-level results have almost halved, from 45% getting five or more grades C or above in 2001 to 23% last year.
As well as academic improvements, the Ofsted report says specialists are offering more extra-curricular opportunities and doing more for their local communities, including adult education.
And it says there is no evidence that neighbouring, non-specialist schools are being disadvantaged by having specialist schools around them.
The School Standards Minister, Stephen Twigg, said: "This report underlines the fact that specialist status drives up standards.
"Over 57% of pupils in specialist schools got five good GCSEs last year compared to 48% of pupils in non-specialist schools."
Specialist schools are not a Labour invention - about 150 gained the status under previous Conservative goverments. But Labour has promoted them strongly.
The shadow education secretary, Tim Collins, said: "This is yet another government flagship initiative that appears to be floundering."