Pupils in most secondary schools in Wrexham are performing well below the Welsh average, according to inspection body Estyn.
It found education services of "significant concern," raising issues from poor attendance to the high rate of exclusions.
Wellbeing and pupils' attitudes to learning in secondary schools compare badly with other parts of Wales.
Wrexham council said it was "on the right journey" for GCSE improvements.
Although pupils made good progress in primary schools, Estyn found poor standards in English and maths at secondary level are a particular concern.
Vulnerable pupils, including those eligible for free school meals and pupils with special educational needs, performed "well below the average" for their peers across Wales.
Though Estyn said there is a clear commitment from the council to improve the situation, its actions had "limited impact" on outcomes for secondary school pupils.
The critical report found:
- A third of secondary school in the county had seen an increase in persistent absence during the last two years
- An increase in fixed-term exclusions and the rate of exclusions over five days is the highest in Wales
- A significant increase over the past three years in pupils attending education outside mainstream schools
- Too many excluded pupils were going without any education, increasing their vulnerability
But Estyn praised wellbeing and the "strong progress" made by pupils in the county's primary schools.
'Scale of the challenge'
Good relationships with families of pupils with special educational needs and support for pupils from the Gypsy and traveller community were also highlighted by the report.
The report comes after the county's head of education, Ian Roberts, said he would be stepping down next year.
Estyn said there is a "clear and ambitious" vision for education but action to improve had been "too slow".
"The implications of weak outcomes in secondary schools in Wrexham are significant for young people's wellbeing and their future education, training and employment", the report added.
"The scale of the challenge to improve outcomes in secondary schools means that Wrexham is an authority that is causing significant concern".
Wrexham council's chief executive Ian Bancroft said the report showed Wrexham had some of the best primary schools in Wales, and there had been improvements in the first years of secondary school.
He urged parents to work with the council as it sought to "translate that into improved performance at GCSE results".
"We'll continue to work with secondary schools and governing bodies to make sure those improvements happen," he said.
"We'll strengthen our leadership of educational services, and we'll continue to make sure that we protect school budgets and invest in education."