Targets to reduce the number of children leaving care without a qualification are not clear or rigorous enough, and some local authorities do not have any.
According to figures from the Local Government Data Unit, 251 of the 397 young people who left care in Wales last year did so without a single GCSE or GNVQ. But the aim was for under 100 to leave without a formal qualification in 2004-5.
Now there have been calls for targets to be tightened up and taken more seriously.
There are currently no national targets in Wales to track the educational attainment of looked-after children, and no plans to introduce them. The Assembly government set a target in 2001 that at least half of children leaving care at 16 would have two or more GCSEs or GNVQs by 2002 and three-quarters by 2003.
That has since been abandoned and officials now agree separate targets with individual local authorities.
But TES Cymru has discovered that four councils which represent some of the most deprived communities in Wales do not have specific targets for looked-after children's education agreed with the Assembly government.
The targets are set out in policy agreements, which show that all 22 Welsh LEAs are tracking the numbers of mainstream pupils who leave without a qualification.
However, Blaenau Gwent, Cardiff, Torfaen and the Vale of Glamorgan do not have separately identified Assembly-agreed targets for children in care - although they may have set their own priorities locally.
Children in Wales, the umbrella group for charities working with children, believes many councils are under-performing in comparison to each other.
In a recent report, the organisation's Mike Lewis said: "For educational attainment at 16, the lowest-scoring authority in Wales scored zero GCSEGNVQ qualification, with the highest scoring just over 80 per cent.
"In terms of educational performance, we know that many looked-after children lack numeric and literacy skills, yet we do not know how many, or which local authorities, are working to improve this situation."
A spokesperson for children's commissioner Peter Clarke said many factors affected the educational performance of children in care, and that setting targets "is only part of the process of ensuring that looked-after children fulfill their educational potential".
But she added: "Targets should be on the ambitious end of the scale, and where these are not already set there is a need for each local authority to agree them with the Assembly."
Janet Ryder, Plaid Cymru's education spokesperson and former chair of Denbighshire's education and family services committee, said she was concerned that those who needed an education the most were being forgotten.
She has asked the Assembly's education committee to scrutinise the current targets.
"There is no one particular person responsible for looked-after children, and it's a big problem getting councils to understand their responsibility as corporate parents," she said.
Conservative education spokesman William Graham said partnership working was the way forward with local authority-set targets. But he said more scrutiny was needed by the Assembly.
"It must have a more definite overview of local targets and must ask each local authority at agreed intervals how these are being met," said Mr Graham.
Both Assembly members have given their full support to TES Cymru's Time to Care campaign, which highlights the underachievement of looked-after children.
An Assembly government spokesperson said: "LEAs should see the improvement of educational attainment of looked-after children as a priority in their role as corporate parents. The Children Act 2004 and Education Act 2005 further strengthen the duties on local authorities and their partners to promote the education of these children."
A spokeswoman for the Welsh Local Government Association agreed that attainment levels for looked-after children were below average but said councils were " committed to improvement".