Bristol has slipped three places to fourth from the bottom in the newly-released GCSE league tables.
More than 31% of all pupils passed with five GCSE grades A* to C, including English and Maths.
The national average for this government-set standard of excellence was nearly 47%.
The city council said the difference between it and other authorities in the tables could reflect the results of as few as 10 pupils.
Councillor Derek Pickup added: "Bristol has secondary schools that parents can be confident in.
"The rapid rise in results achieved at some of our schools proves that with the right leadership, teaching and curriculum, and improved attendance, Bristol can deliver a quality education.
"New facilities have led to dramatic changes in the behaviour and aspirations of pupils such as at the Bristol Brunel Academy. We will continue with the pace of change and improvement to see that translated into better GCSE results across the board."
In North Somerset, the percentage of pupils achieving five grade A*-C passes rose for the fifth year running to stand at 48.5%.
North Somerset councillor Jeremy Blatchford, said: "I am delighted that North Somerset has once again produced a strong set of GCSE results.
"I congratulate the students on their achievements and I want to applaud the dedicated staff in our schools who do so much to support our young people."
In Bath & North East Somerset, the achievement rate was more than 50%, and in South Gloucestershire it was 45.4%.
South Gloucestershire councillor Sheila Cook, said: "We are fortunate that we have a very dedicated teaching staff in the district who help young people to achieve their potential."
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