Serious doubts about the ability of local authorities to run the new school banding system are starting to emerge just weeks before the scheme is due to be launched.
A leading educationalist has questioned whether Wales's 22 councils have the capacity to deliver any meaningful rank order of their secondary schools when the system starts in September.
The comments from Mal Davies, retired headteacher and former chair of the General Teaching Council for Wales, come after Estyn judged Cardiff Council's education services only "adequate" and placed Blaenau Gwent's in special measures.
Mr Davies, secretary of the NUT's Cardiff branch, said the plan had "fundamental flaws" and councils lacked the expertise to ensure the system works properly.
"Most local authorities have few advisers and therefore are unable to even cover the subjects of the national curriculum," he said.
"More importantly, the educational advisory teams of Wales are populated almost exclusively by members of staff who have not taught during the last 10 years.
"This makes their contribution to classroom practice extremely questionable. I accept that they may see a large number of teachers teaching, but that does not replace the need for them to have recent and relevant experience of classroom practice."
Mr Davies said teachers - the "real experts" on classroom practice - should be seconded to fixed-term advisory posts to help the banding system get off to the best start.
Teaching unions said the specialist support given to schools by their local authorities will be crucial to the success of the system, but some, including ATL Cymru and NAHT Cymru, said that support may be hard to find given weaknesses identified in the recent report by Viv Thomas into the structure of education services.
ATL Cymru director Philip Dixon said: "This is the Achilles heel of the whole banding system. If schools get the same level of support it just won't work and they won't be able to move on at all."
Even Estyn's chief inspector Ann Keane, speaking to TES Cymru, admitted there was a "lack of capacity" in local authorities.
"The Viv Thomas report pointed to a new way to support and challenge schools; there's no doubt it needs to be more focused," she said.
Ms Keane said banding would "systematise" good practice that Estyn had found in the best authorities, like Neath Port Talbot.
The banding system was announced by education minister Leighton Andrews in February as part of his 20-point action plan to tackle underperformance after Wales's poor results in the Pisa (Programme for International Student Assessment) tests.
It is being developed jointly by the Welsh Government and the Welsh Local Government Association (WLGA).
Secondary schools will know which band they will be placed in early in the autumn term and the reasons behind it.
Their position will be based on external exam results from 2010, and updated with the 2011 results as soon as possible.
A primary school model is also being developed.
A WLGA spokeswoman said: "Having a banding structure will allow for greater transparency in the education system for both professionals and parents, as well as providing a clear route for local authorities to support schools in improving standards of education."
The Welsh Government said the process was an "important element" of its school improvement plans, and that giving schools an "accurate, rounded assessment" of their strengths and weaknesses would help focus improvement efforts.
A spokeswoman said: "We agree with Mr Davies that experienced heads and teachers have a lot of expertise to offer to support other practitioners working in the education system in Wales.
"This is why we are already working with local authorities to remodel local and regional school improvement to make greater use of the excellent practice and experience that exists in Wales.
"This includes working with them as they identify appropriately experienced and skilled heads and teachers to work alongside others to identify the most effective ways to improve the outcomes for learners in their schools."