Ministers wanted to set a tough new exam target for pupils in care but were prevented because their department does not know how many achieve GCSEs in English and maths, it was revealed this week.
Kevin Brennan, children's minister, made the admission as he launched a new Children and Young Persons Bill, which will legally require all schools with looked-after children to designate a teacher responsible for their needs.
The legislation, published yesterday, comes after The TES Time to Care campaign helped persuade ministers to do more to help the 60,000 children who are in care in England at any one time. It will also ensure they do not move schools while studying for GCSEs except in "exceptional circumstances" and provide them with pound;2,000 bursaries if they reach university.
Last month the Treasury set a new target to raise the proportion of pupils achieving five A*-C GCSEs including English and maths. But a parallel goal to raise the proportion of looked-after pupils gaining five A*-C GCSEs from 12 per cent to 20 per cent did not specify any subjects.
Christine Gilbert, chief inspec-tor of schools, and the children's charities NCH and Barnardo's have condemned the target as inadequate.
This week Mr Brennan told The TES: "There is no policy reason whatsoever why English and maths should not be included in the five good GCSE measure and I would like that to be the case. We should have comparable targets for all children. But I am told that because of the way the data is currently collected it is not possible."
The Government expects to be able to collect the missing data by next summer. But it will be too late to change the Treasury target that will be in place by 2011.
Mr Brennan, who for nine years was head of economics at Radyr Comprehensive in Cardiff before becoming a full-time politician, said: "While I was teaching, I had very limited awareness of children in care. I think it was ... not really in the centre of my radar as a teacher."
The new statutory requirement for teachers with designated responsibility for children in care is designed to change that.
They will ensure schools allocate enough money to looked-after pupils, act as their advocates where necessary and as points of contact for their carers and professionals.
The Government has used Robert Clack Comprehensive in Dagenham as an example. The Essex school's weekly behaviour management programme group, chaired by the head, has a regular item on children in care at which the designated teacher reports on their progress.
Paul Argent, is designated teacher in Robert Clack's lower school with responsibility for five looked-after children.
"I show them where they can find me and if they have any problems, I am their first port of call," he said. "I try to make sure they don't feel different from any other pupil in school. If I were them I might feel I already had enough professionals sticking their noses into my life. But I would probably like there to be somebody for me to turn to if I needed it."
Mr Brennan said: "The designated teacher will be someone they can talk to and share problems with, a significant stable person in that young person's life in school who can help them if they get into trouble, make sure they are not being excluded and try to nip their problems in the bud."
Mr Brennan called on local authority leaders to do their bit by taking an interest in the academic performance of children in their care and organising award ceremonies to celebrate their achievements.
He noted one example he had seen in Caerphilly in Wales, where the council, schools and BT had provided all looked-after children doing GCSEs with their own laptops, printers and broadband access.
"The real key to a lot of this is stability and having children not just in care but being cared for," Mr Brennan said. "It is about admitting that the state has not been a very good parent for these kids and that we have got to pull up our socks considerably in the next few years."