The National Association of Independent Schools and Non-Maintained Special Schools (Nass) said youngsters were often left in residential schools and virtually forgotten.
The comments came as The TES launched its Time To Care campaign, to highlight problems facing the 65,000 children, including those with special needs, who live in care, inside and outside school, in England and Wales.
Claire Dorer, the chief executive of Nass, said the problem lay when children were sent to placements outside the local authority in which their families live.
"These children need to have regular reviews of their academic, physical and social progress, depending on their individual special needs," she said. "Too often parents and carers turn up to review meetings but representatives from local authorities are not there. This is partly due to a crisis in staff turnover in children's social services.
"It has led to an inconsistency in services for the most vulnerable children, many of whom are living a long way from home, even if they are in the best placements for their needs. It is a national problem that needs addressing urgently."
A survey carried out by The TES six years ago revealed that local education authorities often had little idea about the academic achievements of children in residential care, while others admitted that most pupils who were looked-after did not gain any GCSEs. But while ministers spoke at the time of a scandal, little improvement has been made since.
Only 1 in 16 now gains 5 or more top grade GCSEs in England, while in Wales the figure is 1 in 25. Fewer than 100 each year go to university. Children in care are also nine times more likely to be excluded from school. This is despite a series of government initiatives and targets aimed at raising standards.
In 2002, ministers promised to narrow the achievement gap between children in care and their peers. Targets included raising the achievement of 11-year-olds in care to at least 60 per cent of that of their peers and ensuring no more than 10 per cent reached 16 without taking a GCSE or equivalent qualification.
In 2004 they acknowledged the problems faced by children who changed home and school regularly by aiming to increase the proportion who stayed in the same place by 80 per cent.
The TES campaign aims to raise awareness among teachers and politicians of the poor educational achievement of looked-after children.
* Information available every week in 'The TES', or on the website. Go to www.tes.co.ukblogs then select Time to Care