Education services in Torfaen have been heavily criticised in a report by the school's inspectorate, Estyn.
It found that standards were well below what was expected, with an overall judgement of "unsatisfactory" - the lowest rating given.
Estyn said GCSE performance was among the lowest in Wales but primary schools standards improved in recent years.
Torfaen Council accepted the findings and expects to see an improvements in standards as early as the summer.
The Estyn report said Torfaen council needed to raise standards in secondary schools and do more for school leavers who don't go on to find jobs or training.
In particular, the authority came in for criticism for standards at GCSE level in schools across the county.
The report said that over half of schools are in the bottom quarter for performance in the core subjects of English or Welsh, maths and science.
No school is above average on the proportion of pupils achieving the equivalent of five GCSEs at grade A* to C.
The number of pupils suspended has increased - and are among the worst in Wales - however, the number of overall exclusions are reducing significantly.
The report said that performance in Key Stages 1 and 2 - children aged from five to 11 - have improved and the authority was doing more to monitor school performance and intervene when necessary.
However, attendance in primary schools was found to be "below the Welsh average in every measure and standards in this area fall short of what might be expected".
It means the hunt for Wales' first "excellent" local authority education service goes on as Torfaen is the 10th Welsh authority to be inspected and been given the worst rating.
So far, four councils have been considered adequate, three are good and two require significant improvement.
In her annual report, published last month, Estyn Chief inspector Ann Keane said: "Leaders, including governors, in schools and in local authorities need to play a more active role in tackling under performance more systematically."
Torfaen local authority now has 50 days to produce an action plan to show how it will address the recommendations.
Councillor Mary Barnett, executive member for children and young people, said: "We acknowledge the issues highlighted in this report and accept the findings."
She added that the council was already implementing many new strategies to help tackle the issues but these had had too little time for the impact to be measured.
"The report also highlights many positive aspects of education in Torfaen, and I am particularly pleased with performance across key stages 1 and 2, and the great strides made to improve secondary school attendance," she said.
Councillor Barnett added: "We are now supporting and challenging our secondary schools on an unprecedented level, and have shown we are prepared to use all our powers to secure better outcomes for children and young people.
"As a result governing bodies, headteachers and senior leadership teams in schools are already more challenging of their own performance.
"We are already seeing a shift and expect to see school standards begin to improve as early as the summer."
Torfaen assembly member Lynne Neagle said it was "clearly an incredibly challenging and worrying report" and there were simply no excuses for the kind of failings highlighted.
"It also helps underline the fact that the Welsh government was right to place a renewed and unrelenting focus on standards in our schools.
"Especially in these tough economic times, we simply have to ensure that we are delivering a first-class education to our young people and helping them to reach their full potential."
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