South Gloucestershire Council urged to improve secondary schools
Urgent action is needed to improve the overall standard of secondary education in South Gloucestershire, according to an independent review.
The council said there was "evidence of a widening gap" between local attainment levels and the national average in the past five years.
Report author John Harris said teaching needed to improve and surplus places should be reduced.
South Gloucestershire Council said it backed the report.
Mr Harris said although there were some "good strengths" in South Gloucestershire schools, there was "room for improvement".
He said results at GCSE level had fallen behind other local authority areas and there needed to be "a much stronger focus" on "supporting teachers to ensure their teaching is good or outstanding".
South Gloucestershire has 16 secondary schools with a school population of just over 17,000 and has a surplus of 16.6%.
The report said surplus capacity was a "significant factor" in the underperformance of local schools.
Mr Harris said: "The authority doesn't need as many school places as its currently got and as a result many schools are struggling to be able to offer the right sort of curriculum.
"Some schools have as many as 25% spare places so that puts pressure on them to get the right staff and offer the right curriculum."
The report panel also highlighted the need to improve the provision of post-16 education and said a number of sixth forms were "too small".
Councillor Ian Blair, who is on the education committee, said: "The independent report has highlighted the significant challenges we face and difficult decisions we must take if we are to deliver the high quality secondary education that our young people deserve."
The National Union of Teachers (NUT) said it "congratulated" the local authority "for taking responsibility" to address the situation.