A number of academies fall below the government target
A quarter of 607 English schools the government is threatening to close were graded "good" by inspectors, and 16 were judged to be "outstanding".
Only one in 10 was regarded by Ofsted as needing intervention, analysis by the National Union of Teachers shows.
Ministers are targeting schools where less than 30% of pupils get five good GCSEs, including English and maths.
NUT leader Christine Blower said the analysis showed just how "shocking and random" this arbitrary level was.
Last week the government published a list of 638 schools that fell below this "floor target".
The union was able to check published Ofsted reports on 607 of the schools - which are to get extra funding in a "national challenge" scheme to raise their results.
Of the 31 for which no report was available, 14 - almost half - are academies.
These state-funded independent schools with sponsors drawn from churches, businesses, universities or other organisations, are the government's preferred alternative to what it sees as underperforming schools - but only in England.
But the NUT points out that there are 27 academies on the Department for Children, Schools and Families list of 638 schools.
The union says it cannot be right that £260m of the £400m earmarked for this drive is to be spent on establishing more academies or trusts.
"It seems to me that the government has decided to junk its recent attempts to introduce a measure of sophistication into evaluating schools in favour of a crude headline-grabbing measure to try to show that it is tough on standards," said Ms Blower, the NUT's acting general secretary.
"The support outlined in the National Challenge programme will be obscured by this injustice.
"Teachers and head teachers will be very wary of wanting to join schools that could be threatened with closure.
"The NUT will not stand by and watch the vilification of school communities and the intolerable pressure put on heads and teachers as a result of the government's arbitrary actions."
School closures would be resisted and the union's members would be protected from excessive workload demands, she added.
As well as giving extra support to the schools on the list, the Department for Children, Schools and Families does say that many of the schools are on their way to having results above the 30% level.
Schools Minister Jim Knight said he was not saying these were failing schools - his own children had attended a local school which had a 27% GCSE performance but had been judged "good" by Ofsted.
It needed a little bit more support to get above 30% and was confident it would do so this year.
"But we are determined to tackle failure in the education system where we see it," he added.
Schools on the list feel damaged by what the government has done.
'Outstanding to duffer'
David Airey, head of The Charles Read High School in Lincolnshire, said his was a rural secondary in an area of academic selection - surrounded by three grammar schools and with a high-performing all girls school eight miles away.
"Despite this our numbers have increased by 48% and for the past three years our CVA score has placed us in the top 4% of all state schools in England," he told the BBC.
"In May 2007 Ofsted concluded that we were an outstanding school.
"Last year our CVA was 1031.6 but unfortunately we fell below the arbritary 30% including English and maths target for the first time in several years.
"As a result we have been named and shamed. So from outstanding head to duffer in a year!"
He said the government seemed unwilling to look at the context.
"The local press last week used the words failing and closure in their report. Can you imagine the effect this has had on staff and pupil morale?
"What effect it will have on pupil recruitment in the future?
"It is a pity that Mr Balls' comment ... that "This is not about failing schools, there are a large number of these schools that are high-achieving, successful schools" was not made available to editors of local papers.
"Branding my school as weak is simplistic in the extreme and downright unfair."