Childer Thornton primary, Ellesmere Port, Cheshire, consistently gets results above the national average.
But now it is being told by Office for Standards in Education inspectors that it is not getting enough children to level 5 in national tests.
The judgment comes after comparing level 5 performance to other schools with similar intakes. What is particularly galling for staff is that a comparison of level 4 achievement shows it to be doing well.
Confidential performance and assessment (Panda) ratings from A* to E* compare schools' performance to peers with a similar proportion of children on free school meals.
Gareth Powell, headteacher, said: "Our level 4 results put our Panda rating as A grades throughout but that is not enough for our authority or for Ofsted.
"They have now begun to evaluate our level 5 results. We had 45 per cent of children reaching level 5 maths but that gives us an E grade.
"The impact on teaching staff is demoralising. It is all English and maths and nothing else is evaluated. What about geography and French, what about teaching children to enjoy life?"
Mr Powell and 12 other headteachers have written to Education Secretary, Charles Clarke, expressing their concerns about meeting such ever-moving targets.
The reply from his office stated: "Pandas are not part of the public reporting on schools. The tables and data therein provide a series of general, overall, pictures of your school's performance. The information in the Pandas is therefore for schools to interpret and use as they see fit."
Mr Powell said: "It's fine to say the information is just for schools, but the authority and Ofsted both use these ratings to weigh in and say that we're not doing our job."
The importance placed on Panda is part of a wider trend to look beyond raw test scores. This year, for the first time, every primary could have a public grade from A to E to show parents how much children have progressed between the ages of seven and 11.
But unlike Pandas the value-added bands, piloted last year, compare all schools rather than similar schools.
The top 5 per cent of schools measured by value-added will be graded as A, band B will cover the next 20 per cent, band C the middle 50 per cent, band D the next 20 per cent and band E the bottom five per cent.
The scheme to measure added value was piloted last year and consultation on how the results of the 2003 national tests should be published is due to begin next month.