Schools in Wales promise to improve performance
Councils have reacted to the publication of figures by BBC Wales revealing secondary schools' performance in 2010.
A total of 19% of schools performed worse than expected, while 22% did better than expected.
Denbigh High School, which recorded the lowest score, acknowledged "substantial improvement" in English and maths at GCSE was needed.
The school said strategies were in place to achieve better results.
"We would like to reassure parents that we have made and are making every effort in the school, with continued support from the local education authority, to raise standards in these subjects, " said a Denbighshire council.
"Our aim at Denbigh High School is to secure long-term and sustainable improvements to the education of our young people.
"Whilst these figures reflect on standards in English and mathematics, the school can be proud of its successes in many other curriculum subjects and we continuously strive for improvement across the board."
The council said results had improved substantially over recent years and it has drawn up strategies with individual secondary schools to address the 'dip' in maths and English results last year.
A spokesman said: "The focus over the last 12 months has been on improving standards in these two subjects across the county.
"There has also been an improvement in results of individual schools and improving education across the county remains a key priority for the council."
Of the 22 local authorities, Monmouthshire was rated lowest of all in terms of value added score.
Value added scores are calculated by estimating what a pupil is expected to achieve at GCSE, including the core subjects of English or Welsh and mathematics, when they enter the school at 11.
Andrew Keep, Monmouthshire's chief officer for children and young people, acknowledged the results were "disappointing".
He said: "We do need to improve and that's something that is recognised by the schools.
"Up until 2007 the Level 2 performance of Monmouthshire had been largely static, but following some specific actions, improvement from 2007-2010 rose by 10.6%, from 54% to nearly 65%."
He said that exceeded the improvement made across Wales during the same period.
"There's much to be done and we are working actively with schools to ensure the trajectory of our improvement is continued and we address the value added issue that these figures identify."
Pembrokeshire, which came third from bottom, also said it was disappointed with the 2010 GCSE results but was doing "everything in its power" to raise standards for 2011.
However, the council said it believed the performance of its education service could not be judged by one statistical indicator alone.
"The data used by the BBC also looks at the performance of just one cohort of pupils - those in Year 11 in 2010. The overall picture could vary significantly if a different cohort was examined.
"The authority concurs with the view of the minister for education and skills that simplistic league tables can be divisive and misleading and do not, in themselves, promote improvement. "
This view was shared by Ceredigion council, which came fourth from bottom. Its officers have spoken to Education Minister Leighton Andrews about the figures.
A spokesman said: "Insofar as Ceredigion County Council is concerned, it has never concealed the priority of its desire to improve educational provisions across the entire county.
"If the statistics are indicative of anything then they surely show that the county council is correct in giving such a high priority to that which needs to be done - improving the educational provisions across the entire county of Ceredigion."