Successful schools are being pressured into agreeing over-ambitious improvement targets by local authorities desperate to meet their own Government-imposed targets, headteachers claim.
Local authorities have been given tough targets for the number of 11-year-olds achieving the expected level in literacy by 2002, with similar arrangements for numeracy expected in the summer.
Negotiations are under way between schools and local authorities to ensure that the overall targets can be met. Schools will have to publish their own performance targets in September.
"Schools doing very well, at full stretch, are being asked to meet ridiculously high targets to compensate for those that are not going to be able to meet their targets, so that the local authority average is hit," said Arthur de Caux, senior assistant secretary of the National Association of Head Teachers.
"It seems to move responsibility (for target setting) from schools to local authorities, and has brought greater pressure from authorities on to schools."
The NAHT will discuss the issue at next month's meeting of its curriculum and assessment committee.
Mr de Caux said the number of calls it had received from association members suggested it was an issue "that is going to grow".
The association has written to branch secretaries reminding them that the responsibility for setting school targets lies with schools, and urging heads not to be pressured into accepting unreasonable demands from education authorities.
David Whitbread, the Local Government Association's head of education, foresaw problems reconciling school and authority targets.
"Every local authority is said to have agreed and volunteered to their target, but the Government has certainly wanted them to be challenging," he said.
"Whatever pressure authorities are under gets passed on to the schools, and it is a matter of judgment whether the school targets are too challenging. If the NAHT is suggesting some schools are being given over-challenging targets, it has to ask does that mean others are being given under-challenging ones?" Nor is the draft code of practice - which is meant to set out the formal relationships between schools and authorities - going to help, according to Ian Aspden, Cheshire's education policy manager.
He added: "There are schools that don't like what appears to them to be a top-down approach coming from government.
"We have given our schools indicative figures which, if they adopt them, will come to the total for the county. There is no indication that schools are not going along with the whole thing, but some will have difficulties achieving those targets - they could just be having a bad year."
Richard Riddell, Bristol City Council's education director, said it was "not going to get too heavy" about hitting its own key stage two target in the first year - although it will be challenging high-achieving schools to do more for their children, if it thinks they humanly can.
"We haven't seen whether all the school targets add up to the authority's target, and we are not going to have a panic about it," he said. "We have to stick by what is reasonable and achievable."