GCSE results at Bolton Muslim Girls' School slipped by eight per cent this year - but new government league tables revealed staff at the school were still doing a good job.

Alongside traditional tables, which rank schools according to GCSE scores, ''value added'' data has been published for the first time - in a pilot scheme involving 200 schools.

These tables judge the performance of pupils in their GCSEs compared to their Key Stage Three results.

These in turn are judged against results from Key Stage Two tests, which are taken by youngsters at 11 years old.

The success of the school at each stage is expressed as a number, where anything greater than 100 shows the school performance is higher than average and lower than 100 is poorer - based on the abilities of the pupils in the year group.

So at Bolton Muslim Girls' School, although GCSE results were lower than last year, the value added measures of 103 and 108.7 showed the school's performance had improved more than average.

Welcoming changes

Headteacher Ibrish Patel has welcomed the value-added tables because they are as much an indication of progress as performance. And he believes they are a useful indication for the school.

''These value-added tables show whether the children carry on at the same rhythm and the same progression,'' he said.

''That's the reason I think the pilot has been very successful.

''It gives a real picture of what is happening at each and every stage.''

However, David Watchorn, head at Abraham Moss High School, in Manchester, is more sceptical about the calculation of value-added.

In his school, the Key Stage 2-3 measure is 98, but the Key Stage 3-4 is 103.1.

Improvement

Mr Watchorn says that in reality, he believes there is a steady improvement throughout a youngster's five years at the school.

He said: ''I don't think it has been very successful at all to be honest. There are too many variables that are not taken into account.''

He says the high amount of mobility of youngsters at his school means that many arrive without the baseline marks for Key Stage Two or Three necessary for them to be counted at all in the tables.

Yet he believes that to be a successful indicator of performance the tables need to include the achievements of all pupils.

In addition, he says there will be high numbers of pupils who have special educational needs or speak English as a second language, whose achievements may be significant, but not enough to be measured on the tables.

''My concern is that value added tables aren't going to be necessarily any more valid that what we have now,'' he said. ''And I object to them publishing tables that the public might think are more valid.''

According to the Department for Education and Skills, for schools with more than 100 pupils value added scores of 99.3 to 100.7 are broadly average for Key Stage Two to Key Stage Three value-added measures.

And when looking at data for similar schools between Key Stage Three and GCSEs, the figures of 98 to 102 are broadly considered average.