JANET DOBSON has reported on headteachers’ views about pupil mobility and its effect on school performance (TES, February 5). She is quoted in the Department for Education and Employment’s new action plan, Excellence in Cities, which acknowledges that pupil mobility in inner cities is often high.

But how high? How many mobile pupils are there in a local authority? And how does mobility affect the performance of LEAs, as distinct from the performance of individual schools?

For LEAs, pupil mobility can be a critical issue. Since 1996, Hackney has measured the effect of mobility on pupil achievement and school performance as a whole. We believe that much of what we have learned applies to other LEAs. Hackney’s position is not unique, though it is probably at the extreme end of the spectrum.

How many mobile pupils are there?

When they are tested at key stage 1, one in five of Hackney’s seven-year-olds has not been at the same school for the full KS1 curriculum. At KS2, one in three of our Year 6 pupils has not been at their school for the full four years of KS2. One in five of our Year 9 pupils taking KS3 tests was not at the same Hackney school at the beginning of Year 7.

Typically, more than a quarter of Hackney’s GCSE cohort joins their school during Years 10 or 11, after the GCSE courses have already begun.

These LEA averages mask the extremely high mobility rates of individual schools. And they omit the many children who come, and then go again, before being tested at the end of a key stage.

Who are the mobile pupils?

Our research shows that the mobile pupils - the new arrivals, the mid-term entrants - are more disadvantaged than the general school population. They are more likely to be eligible for free school meals. A high proportion have English as an additional language. Many are complete beginners in English. They are more likely to be from ethnic groups which currently achieve less well than others. In the secondary phase, more of them are boys. Taken overall, these new arrivals depress the average test results of schools and the LEA.

What do mobile pupils achieve?

A close look at Hackney’s 1998 KS2 results is illuminating. One third of the KS2 pupils had arrived in Years 3, 4, 5 or 6. The pupils’ test results mirrored closely the amount of time they had spent in the same school. Pupils who had arrived in Year 6 (the same year they were tested) fared very badly. Year 5 arrivals did slightly better, though less well than those who joined during Year 4. The generally lower scores of new arrivals distorted the LEA average. This masked what was achieved by children who had been in the same Hackney school for the full four years of the key stage. (See box).

Hackney’s KS2 results placed it fifth from the bottom of the league tables. Yet the aggregate scores of the pupils for whose education particular schools were wholly responsible was above that achieved by 38 LEAs. A flawed comparison, certainly, since other LEAs have mobile pupils too. But perhaps less flawed than the league tables as they stand.

The starkly lower achievement of mobile pupils has a major impact on the LEA’s performance levels for secondary pupils, too. (See box).

We tend to think of schools as stable communities in which children stay, grow and learn. But they are only as stable as the communities which they serve. For LEAs, the effects of high pupil mobility can be wide ranging. Pupil turnover has a negative effect on an LEA’s attendance figures, its target-setting and the planning of school places. It clouds the inspection judgments of its schools, invalidates cohort-level value-added analyses and makes unpredictable demands on its specialist support services.

But pupil achievement and school performance are the central issues, because raising standards is our core task. To fulfil its strategic role in school improvement, the LEA has to untangle the complex web of pupil mobility and its differential effect on the performance of individual schools.

We need to explore further the factors which cause mobility. We need to understand more about the assessment and support strategies which might reduce its effects.

Accountability is a cornerstone of current education policy. Rightly so. But how accountable is a school or an LEA for the test results of children who were not there to be taught?

Colin Alston is head of research and statistics in Hackney LEA and a member of the advisory group on the pupil mobility in schools project, migration research unit, University of London