The latest drive to improve Oxfordshire schools is to be targeted at 'coasting' schools which appear to be performing well, rather than those at the bottom of league tables.

Using data produced by the independent education organisation Fischer Family Trust (FFT), the county council is trying to improve schools which do not seem to be reaching their potential. The FFT estimates how individual pupils should perform by taking their prior attainment into account, along with factors such as gender, ethnic background, special educational needs, free school meal entitlement and postcode.

According to the FFT estimates, only two county secondary schools exceeded expectations at GCSE last year - The Cherwell School, Oxford, and St Birinus School, Didcot - while 14 performed below and 18 in line with expectations.

Test results for 14-year-olds presented a more positive picture, with seven schools exceeding expectations at Key Stage Three.

Some heads have criticised the figures, but Michael Waine, cabinet member for school improvement, said they were vital for challenging under- achievement. He said: "It's not an exact science, but it's the best science we've got. If we're going to make judgements with schools in terms of the efforts they are making, we need to operate on a fair playing field. This puts it onto that playing field."

Council senior schools adviser Roy Leach said: "There has been a real emphasis on the lowest attaining schools in the drive to raise achievement - this is a shift away from that and a recognition that if you are going to significantly raise the overall levels of attainment, you are not going to do it by focusing on schools like Peers and Drayton because their capacity to bring about improvement is rather less than some schools, which on the face of it are doing well."

But Patrick Sanders, head of Burford School, a high-performer which FFT says could do better, said: "It's just one piece of information that you take with a pinch of salt.

"We are not underachieving, we are doing very well and the vast majority of statistics show that."

And David Wilson, head of Faringdon Community College and chairman of the Oxfordshire Secondary Headteachers Association, warned schools which offered more GNVQ options appeared to do better than schools with a tougher, more traditional curriculum. He said: "It worries me if the local authority thinks this is the best way to make decisions."