The Vale of Glamorgan council is the latest to fall foul of the education watchdog in this recent round of inspections in Wales.
Currently five local education authorities have been deemed “good”, six “unsatisfactory” and nine “adequate”.
A report on Swansea is due at the end of this month while the final of the 22 reports will be published next year.
In the latest assessment of education services in Wales, Estyn said the Vale of Glamorgan did not evaluate provision and leadership in schools well enough and relied too heavily on performance data to categorise schools.
It added that it had failed to address recommendations from its previous inspection three years ago and benchmarked performance in all key stages was weaker in 2012 than it had previously been.
The Vale of Glamorgan council said in response that an action plan had been drawn up and it was intent on ensuring improvements.
The report comes as education minister Huw Lewis wrote to schools in Wales to make clear to teachers his “commitment and support” to improving standards.
The former chemistry teacher said he was familiar with the “pressure, excitement and opportunity” that comes with a new school year.
He said: “The stable set of results we saw in GCSEs and A levels, closing the gap in certain key areas with England, are testament to your hard work. There is much more to be done, however.
“We all have our part to play in improving the education system in Wales, and government is no exception. It is therefore vital that we provide you with the right support, funding, tools and resources to deliver our key priorities.”
He also acknowledged that there had been a lot of change and added that he wanted to ensure that with the change came the right support.
As part of that pledge he announced that this term there would be a series of events to launch the Welsh government’s review of qualifications including a conference in Cardiff on December 11.