A fixed quota of secondaries will be judged to be below crucial "floor standards" and eligible for closure or other intervention, regardless of how high overall exam results climb, under a new proposal from the Westminster government.
The idea, contained in its secondary school accountability consultation, is causing consternation among union leaders who say that it could leave some headteachers' jobs on the line even if their results improved dramatically.
Other measures proposed in the biggest overhaul of secondary league tables since their introduction in 1992 have been broadly welcomed. They include an end to the current benchmark of five A*-C GCSEs including English and maths as the main headline measure, and the introduction of a sampling system as a new national measure of standards.
Schools which do not meet floor standards are open to a variety of government interventions, including closing them and replacing them with academies, with heads losing their jobs. At present, the standards are based on predetermined targets for exam results and pupil progress levels. If the targets are cleared, then schools have nothing to fear.
But Department for Education (DfE) officials are concerned that they will not know in advance where to pitch the targets when revamped GCSEs are first sat in 2017. Instead, they are suggesting that, for one year only, a fixed quota would be set. Schools' actual results would not be a factor in floor standards - just where they were ranked in relation to other schools.
"In this approach, we would identify the worst-performing number of schools, rather than those below a predetermined floor standard," the DfE consultation says.
Russell Hobby, general secretary of the NAHT heads' union, said he would oppose the quota idea. "It doesn't actually make the case for if a school is good or bad," he said.
"It just makes the case for whether it is a better or worse school. Unless there is some guard, some kind of absolute criteria, then I think people will be very unhappy with that kind of approach.
"Even if you had a group of absolutely outstanding schools, one of them would be worse than all the others, but you wouldn't be justified in shutting that school down on that basis."
The DfE consultation appears to acknowledge this problem with its own proposal, saying: "We do not propose to use relative measures as part of the longer-term floor standard arrangements as this would mean each year there would be a set number of schools who fall below the floor standard, with no opportunity to recognise improvements across the system."
After a year of the new GCSEs, ministers intend to base floor standards on two new measures (see panel). They are also proposing a new system of national sample tests in English, mathematics and science. They would be similar to the international Pisa, Pirls and Timss tests, but would take place annually.
The current five A*-C GCSEs including English and maths measure will be replaced as the main headline measure and basis for school "floor standards".
Floor standards will be calculated on two new measures.
The first will look at the proportion of pupils in each school that achieve a C or equivalent in English and maths GCSEs. The second will be based on a points score indicator set according to pupils' achievements in eight qualifications, with credits given for every grade.
English and maths GCSEs will be compulsory in the measure, alongside any three of the other EBac GCSEs: languages, computer science, history, geography and sciences. The three remaining "slots" could be filled by further EBac GCSEs "or any other high value arts, academic, or (government approved) vocational qualifications".
School floor standards will look at how much value has been added when pupils' achievements on the points score measure are compared with their primary test results.