Schools are accused of concentrating their effort on pupils most likely to do well, at the expense of those deemed to have little prospect of getting the best marks.

David Hart: "We need to reduce the long line of underachievement"

The point is given added bite this year with the results showing a slight rise in the numbers of entries attracting grades A* to C but a decrease in the overall pass rate once the lower grades are brought into the equation, and a bigger rise in the numbers getting an unclassified U grade - a failure.

Grades A* to C are equivalent to the range of grades in the O level which the GCSE superceded 10 years ago.

The league tables do also show the proportion of pupils in a school getting at least five grades A* to G - in other words, all pass grades - and those with at least one pass grade. But because they have a separate column for grades A* to C, that is what everyone tends to look at.

The government is proposing to introduce a new 'points score' for GCSEs as it has for A levels. This system produces an average point score per examination entry in a school and does not depend on the number of examinations taken - but it will sit alongside the grade figures, not replace them.

The General Secretary of the Secondary Heads Association, John Dunford, said: "As long as A* to C remains in league tables, that will be the measurement people use and that is damaging, because it invites schools to concentrate on one particular group of children.

How the government league tables appear

"Schools don't want to do this, but those in particularly competitive situations, and those which have had poor inspection reports, feel forced into it."

The National Association of Head Teachers is drawing attention to the rise in the proportion of papers which failed to get a grade G this year, from 1.5% in 1997 to 2.3% - a total of 123,000 exam entries.

The union's General Secretary, David Hart, said: "There is no escaping the fact that the gap between the majority, who are achieving very good results, and the small minority, who have nothing to show for their efforts, is getting wider."

"The government must radically reform both the performance tables and its secondary school targets so that they reflect the performance of all pupils, otherwise they will reinforce failure and increase an education underclass."

These concerns are echoed by the Association of Teachers and Lecturers. Its Deputy General Secretary, Gwen Evans, said: "The competitiveness fostered by league tables has pushed schools into concentrating resources on those who are likely to get the best grades."

John Dunford: "Government policies make teachers do things they don't want to do"

The Director of the School for Education at Brunel University, Professor Alan Smithers, said: "League tables treat the C grade as if it were a pass, and do not recognise any level of performance below it.

"Quite naturally, schools put their effort into achieving passes at that level, which neglect nearly half of all our pupils. That is showing up in the poorer performance at the lower end of the scale.

"If the national targets include A* to C grades, that is what people will continue to use, and the new points score will be ignored. This is not just an accounting operation or a way of expressing data. It has a very important effect on the way schools operate, and we can see that in these results."

A senior government source dismissed calls to abandon the A* to C grade measurement.

"We are not going to withdraw information from the tables which parents and employers understand and which they are used to receiving," he said.

"We are providing more information, not less. The A* to C grade measure is well understood and well appreciated. It is a goal for schools to strive at, as well as ensuring that all pupils achieve exam success."