Just over half the children looked after by local authorities are entered for only one GCSE, it has emerged.
Individual help for those in care would help to raise results, the NCH says
Education Minister Maria Eagle said she was "shocked" by the figure of 54%.
David Kidney MP, who chairs the All Party Parliamentary Group on Children in Care, said it was "outrageous".
There are some 61,000 children in care. Ministers say they aim to close the gap between their attainment and that of other children "substantially".
Labour MP Mr Kidney said: "We have had an outrageous situation where achieving one GCSE has been a target for children in care for a long time.
"This is a poor and very low requirement."
But the Department for Education said it had never had a target for children in care to achieve one GCSE - it simply collected statistics on the proportion of children who achieve this.
Mr Kidney said children in care often had difficulty getting into good schools, despite government guidance which emphasised that schools should make children in care a priority.
"This is one of the many barriers children in care face.
"I have heard of children in foster care who have waited four or five months to find a school because some do not want to take them.
"Ofsted really should be exposing those schools who do not want to take in these children."
Mr Kidney added that the Department for Education and Skills should be focusing on implementing the existing guidance for schools - new laws were not needed.
A report by the children's charity NCH says that in 2004, 6% of children in care achieved five or more GCSEs at grades A* to C or the equivalent, compared with 53% of children overall.
Of care leavers, around 43% achieve at least one GCSE or GNVQ, its research says.
There are around 61,000 children in care, of whom 55% are boys, according to the NCH.
Its policy director, Caroline Abrahams, said one GCSE "would not take children very far".
But she said higher targets would not necessarily remove the barriers to higher attainment.
"The government needs to tighten up admissions so that children in care are not simply put in the worst schools."
Catch-up help would improve their chances of getting good GCSEs, she added.
A "significant number" of children in care are still not entered for any GCSEs at all, she said.
A more flexible education system would give them a second chance to catch up if they missed out on taking these qualifications.
The Department for Education and Skills says that by 2006, children in care should be achieving results which are at least 60% as good as those of their peers.
This would mean 31.8% of children in care achieving five or more good GCSE grades.
A spokeswoman said the proportion of those aged 16 who got qualifications equivalent to five GCSEs at grades A*-C had risen by four percentage points each year since 2002, and that across all local authorities 15% of children achieved this.
The department says it wants to "substantially narrow the attainment gap".