The new primary league tables may have come as a nasty suprise to parents who have fallen over backwards to get their children into the 'right' school. Judith Judd explains why
Take the family Bosworth. Two years ago they left east London for a Wiltshire village, solely for the benefit of their children's education. Instead of a tough, London primary school, full of difficult pupils and left-wing teachers, they reasoned, their children would be educated by kindly staff in a cosy village school.

Yesterday morning, the Bosworths had a shock. The Hackney school they deserted far outscored the Wiltshire school in the Government's first primary league tables.

The neighbours who had bid them an anxious farewell, secretly wondering whether they were short-changing their children by staying put, were smug. At their local school, they observed, the percentage of pupils reaching the expected standard or better in English, maths and science at the age of 11 was well above the national average.

The Bosworths are imaginary but their plight is not. Throughout England the perceptions of puzzled parents are being overturned. Fashionable schools which are all the rage at the school gate appear, in some cases, to be performing less well than their less popular neighbours. Parents in some areas seem to have been fighting ferociously to get their children into the wrong school.

So what do the league tables tell parents and how should they be read? Gillian Shephard, the Secretary of State for Education, says that they shed light, for the first time, on the nation's primary schools. If that is so, it is a very murky light. The most unequivocal message is about the type of intake. On the whole, the better-scoring schools are those with a reasonable proportion of bright, middle-class children. All the top-scoring schools are in villages or suburbs.

Those at the bottom of the heap tend to include large numbers of children from families so poor that they are on free school meals. One Birmingham school in the bottom dozen has 75 per cent of its children on free meals, compared with a national average of 19 per cent. It also has a catchment area in which pupils are constantly on the move.

Disadvantage is not confined to the inner cities. Schools in some rural areas may have to work just as hard to bring their pupils up to the required standard. Inner-city schools don't have a monopoly of poor or below-average children.

So what the tables do suggest to parents is the social background of their children's classmates. And for most, for better or worse, that is important.

But it is not the whole story. Parents also want their children to be taught well and enthusiastically. The tables offer very limited help here. Unless the ability of children when they start at a school is taken into account, it is impossible to measure how well the school has taught them.

That is why both the Government and Labour are committed to testing children at five and then publishing "value-added" tables which will measure the difference individual schools have made to children's progress. These will be tricky to compile but much more useful to parents than what was published yesterday.

At present, there is nothing to tell parents which schools with a reasonable share of able pupils are getting the best out of them and which are sitting comfortably on their league table laurels, achieved simply because they happen to have a more intelligent intake than the school down the road.

Even the top school in an area may not be doing as well as it should. The tables give the proportion of pupils reaching Level 4, the expected standard, and above. They don't say what proportion of pupils reach Level 5, which proves that a child is two years ahead of the expected standard. Parents of bright children will want to know. A school which is good at bringing most of its children up to Level 4 may still be failing its high- fliers.

So the league tables don't expose failure. They mask it. They also mask success. Some inner-city schools, shunned by the middle classes, are doing a very good job with pupils of below-average ability.

Of course, parents can make guesses about how their local schools are doing in relation to their intake. Most people have a rough idea what sort of children go to which school. X has more from the council estate than Y. Z is close to all the big houses in the professional people's ghetto and is mentioned in the estate agents' blurb.

In a few cases, the league tables may reveal surprising differences: the school the middle class avoid turns out to score more highly than the one they all favour. But mostly, parents have to rely on guesswork and their local knowledge to decide who is really doing best.

The other difficulty is that a single year's figures may not mean much. Many primary schools are small - 1,500 had 10 or fewer pupils eligible to take the test and were left out of the tables altogether. Thousands have fewer than 20 eligible pupils. That means that results are likely to vary sharply from year to year according to the ability of children. A couple of children with learning difficulties may make a big difference.

The same is true of absences. The percentage reaching the expected level is calculated using the number of pupils eligible to take the test, not on the number actually sitting them. Nor does it take into account the children with special educational needs. Both may depress schools' results considerably.

So should the Bosworths have moved? If they had stayed, they might have struggled to secure a place at the high-scoring London school as everyone else tried to do the same. After yesterday's tables, competition will be even fiercer.

And their village primary school may be teaching its children well even though it has a lower proportion of able children than its opposite number in Hackney. Or perhaps several children were sick during test week? Or perhaps it just had a bad year. The frustration for the Bosworths is that, despite the league tables, they will probably never know