By Gary Eason
Education editor, BBC News website
Labour's 30% "floor target" for attainment in England's secondary schools is the latest in a series of demands related to the notion of "five good GCSEs".
The requirement - announced by Prime Minister Gordon Brown last autumn - is that at least three in every 10 pupils in each school achieve at least a grade C in five GCSE or equivalent qualifications, including English and maths GCSEs.
On the most recent exam results - those from last summer - this catches out 638 schools in 134 of the 149 local authorities in the country that have state-maintained secondary schools.
Click here for a table of the 638 schools
The arbitrariness of the target can be seen in the fact that, on a different reading, 631 schools are affected.
This is because the government uses two measures of secondary school performance: attainment by those aged 15 at the start of the school year, and attainment by those at the end of Key Stage 4 of the national curriculum - which can be a little later.
Its preferred measure is usually Key Stage 4, but for this target it is using 15-year-olds - so seven more head teachers are feeling the heat.
(The 15-year-olds are of course almost all 16-year-olds by the time the results are published each August).
The difference confused schools - a number contacted the BBC News website to say their published results were incorrect, not realising which precise measure the government was using.
The head of the National Union of Teachers, Christine Blower, said: "I cannot understand the logic of saying that 29% of pupils with five GCSE A*-C represents failure and 31% signifies success."
But the government is quite deliberately "raising the bar" in an effort to drive up performance and, in Mr Brown's words, "eradicate failure".
After Labour took over the government, in October 1998, GCSE targets were set for the first time.
The aspiration then was that by 2002, at least half of 16-year-olds nationally would be achieving five good GCSE passes (in fact 51.5% did so).
Then in March 2000 a new challenge was set relating to each school: that at least 25% of its pupils would have five good GCSE passes by 2006.
The then education secretary, David Blunkett, said there were 530 schools on or below that target level.
At the time, the "five good GCSEs" (or vocational equivalents) did not include English and maths.
In fact at first things were made easier for schools by the inclusion in the definition of "equivalents" a much wider range of qualifications.
But they were made significantly harder for many when those core subjects of English and maths became part of the benchmark, in the 2006 performance tables.
This proved a significant hurdle for some schools which had apparently been doing well on the previous measure, under which any five GCSE subjects counted.
Last year nationally 46% of pupils managed the new "gold standard". But in 638 schools, less than 30% did so.
And it was those schools on which Mr Brown turned his guns last October, in a speech in which he promised to "eradicate failure" from the system by 2012.
Their local authorities are now being given until the end of this term to come up with action plans to make sure this happens.
Fifteen authorities have nothing to do, therefore.
But in some areas most schools fall below the 30% threshold.
It raises the question whether the government can be serious about carrying through the ultimate threat to close those which do not make the grade.