Schools near army, naval or air force bases have long complained of the difficulties they face when confronted with often staggeringly high rates of pupil turnover.

Now a group of 90 heads have banded together to persuade the Government to recognise that fact in benchmarking and target-setting calculations, and to campaign for a national funding formula.

The National Association of State Schools for Service Children believes central government grants to councils should take into account how many children from service families a school educates.

Currently, those grants factor in how many pupils are eligible for free school meals and the numbers using ethnic-minority support services, but there is no measure for pupil turnover.

Yet “service schools”, which can see up to two-thirds of their pupils change every year, say this adds significant costs, including that of assessing new children.

Their concerns are adding to the clamour for ministers to recognise the effects of pupil turnover - or “turbulence” - on achievement. Last month, government-sponsored research by University College, London, claimed that pupil mobility damaged standards in many schools.

The association’s chairman, Val Clark, head of St Michael’s primary school, Colchester, said all schools should receive support of the kind hers enjoys from Essex County Council.

Following a campaign by eight armed-forces schools, the authority now has a formula whereby every 2.5 service children qualify for extra funding equal to that of one “free-meal” pupil. The funding allows her school, where 95 per cent of pupils are from army families, to pay for teaching assistants in every class.

Mrs Clark said: “This is vital, because we are continually having to assess children as they arrive, and to help them fit in in a class.”

The association claims that services schools, and others with high levels of turbulence, will again appear in a bad light in next week’s league tables.

It is calling for target-setting for school year groups to be dropped where high turnover rates make those targets “meaningless”. It also thinks schools with high turbulence should only be compared with similar schools when benchmarking.

And it plans to work with the armed forces in trying to ensure that pupils do not have to move schools at sensitive times, such as during curriculum tests or GCSEs.

The association, which is seeking a meeting with ministers, was launched at a London conference organised by the National Association of Head Teachers yesterday.

David Hart, NAHT general secretary, said: “Whenever we have argued that turbulence should be recognised as a factor in performance, schools for service children have been ignored.

“It is time they are listened to, and I’m sure their concerns will be looked at sympathetically by ministers.”