An analysis of the English test results of nearly 1,000 children suggests that the marking system is plagued by inaccurate grading.
One in seven pupils at a group of 17 Essex primaries had their national curriculum writing levels changed on appeal in 2004 and 2005, research by a headteacher reveals.
Stephen Chynoweth, head of Tyrrell's primary in Chelmsford, now wants wholesale changes to the marking of English tests and regional marking centres to be set up. He said poor marking not only damaged schools, but also pupils.
"If a child gets a result lower than expected, their confidence, self-esteem and how they think they will do in secondary school will all go down," he said.
Many heads have complained about the standard of marking, but Mr Chynoweth is among the first to collect evidence from a sizeable group of schools.
He spent 18 months collecting the statistics after his school's results in the writing test fell from 80 per cent at level 4 in 2003 to 52 per cent in 2004, even though teacher assessment suggested they would be in line with the previous year.
The school sent all 59 papers to be re-marked. From these, 25 pupils had their writing level changed. A further 30 received a change of mark, but no change of level. These changes pushed the proportion achieving level 4 up to 78 per cent. Clerical errors by the marker also accounted for some incorrect levels.
Mr Chynoweth then collected data from 16 other primary schools and found that the writing levels of 185 out of 848 Year 6 pupils were increased on appeal.
Barnes Farm school, Chelmsford, challenged the 2004 results of its 76 pupils and 20 children (26 per cent) had their writing levels upgraded.
Tim Barrett, the head, said: "The quality of marking is just not good enough."
The heads' concerns have been backed by advisers at Essex council who wrote to Mr Chynoweth agreeing that the marking in 20045 had been plagued by a "wealth of clerical errors".
Mr Chynoweth, whose school has been described by inspectors as outstanding, has discussed his findings with the National Assessment Agency. He was told that one in 70 markers nationally was "ineffective" but believes the figure must be far higher.
A spokesman for the Qualifications and Curriculum Authority highlighted national figures for 2005 suggesting that far fewer than one in seven pupils had levels changed on appeal.
He said that in Essex as a whole some 50 out of 6,000 pupils had their overall English level changed on appeal, but he was unable to provide figures related only to the writing paper.
Mr Chynoweth would like to hear from other schools that have had bad marking experiences.
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