The new key performance measure for secondary schools is likely to lead to the neglect of less able students, according to an academic who helped to devise it.
Professor Simon Burgess says there is good reason to think that Progress 8 - which will replace the benchmark of five A*-C grades at GCSE including English and maths from 2016 - will incentivise schools to "ignore the low-performers".
Schools might be encouraged to focus their efforts on the most able pupils in the belief that this would be an easier way to do well on the measure, "increasing the polarisation of attainment" between students, the University of Bristol economics professor told TES.
The academic, who was hired by the Department for Education (DfE) to advise on the measure, also warned that it would be skewed against schools with higher proportions of disadvantaged children or boys.
The warnings are in stark contrast to the aims of Progress 8, which was designed to make the system fairer by removing the focus on the CD borderline. Instead, it gives schools credit for all the grades that pupils achieve, looking at whether they have met expectations in relation to their prior attainment.
DfE officials commissioned Professor Burgess to come up with a way of ensuring that institutions with high-ability intakes, such as grammar schools, were not given a built-in advantage.
The academic believes this has been achieved, but is concerned that the government has decided to set high-stakes floor targets based on the average level of student progress. He said this could allow schools to "mask" underperforming students by boosting the grades of their most able.
"It may be that raising the average is easier by throwing resources at the higher-ability pupils," Professor Burgess told TES. "If it is easier to raise a B to an A than a D to an E, then that is what you'll do."
He said schools that had to respond quickly to meet targets were likely to focus on pupils they knew had "a high ability to learn". "If you were highlighting a school for having some pupils not doing very well, you would want it to address the problem with those pupils and to put some more effort in to try and raise their grades," Professor Burgess said. "But as the system is an average over the school, that doesn't necessarily follow."
Keith Grainger, principal of Garth Hill College in Bracknell, Berkshire, said: "I think it would be very sad if a single school in the country chose to concentrate on one group of pupils at the expense of others. I can't imagine they would. We want all our pupils to progress and achieve. Generally we welcome the introduction of the Progress 8 measure. It is better than what we have had."
Professor Burgess added that the numerical grading system being phased in for GCSEs from 2017 - replacing A*-G grades with numbers 9-1 - could further encourage schools to concentrate on high-performing pupils as there will be a greater number of grades at the top of the ability range.
The DfE has decided not to include factors such as gender or socio-economic disadvantage when calculating the progress measure because it does not want to lower expectations for particular groups of pupils.
But Professor Burgess said this would mean that "schools with lots and lots of boys are going to find it harder to hit that average target than schools with lots and lots of girls. Schools with lots of poor kids are going to find it harder than schools with hardly any poor kids. The DfE is fully aware of that."
Russell Hobby, general secretary of the NAHT headteachers' union, said that Professor Burgess' fears were based on "the assumption that it would be easier to get a student from a B to an A, for example, than from an F to an E".
"I don't know if that is proven because you might also say that the B student could be already performing to the top of their intensity whereas the F student might have a lot of untapped potential," Mr Hobby said. "This measure certainly loosens schools up to do what's right. You would hope that they would take a moral position on this one."
A DfE spokesman said: "Our new accountability system will ensure that schools, particularly those with challenging intakes, are getting the credit they deserve for helping all pupils achieve their potential."
`Why do we do what we do?'
Keith Grainger, principal of Garth Hill College in Bracknell, thinks the issues raised by Professor Simon Burgess pose both moral and practical questions for schools.
"On a practical level, Progress 8 will be the headline measure but it will be accompanied by several others. So if school chose to prioritise support for a particular group of pupils to the detriment of other pupils, that will still come out in other measures," he says.
"Quite rightly, these would lead to some challenging questions, including those from inspectors. On a moral level it comes down to one question: why do we do what we do? Surely, as educationalists, we are trying to do our best for all our pupils."