Inspectors criticised teachers at the Business Academy in Bexley and said the south London school was suffering significant weaknesses.
The conclusion was reached even though the academy, described by Prime Minister Tony Blair as a "beacon of hope" when he opened it last year, has seen massive improvements in exam results.
Now millionaire sponsor Sir David Garrard, a property developer and chairman of governors, is taking legal advice and will seek a judicial review of the way the inspection was carried out.
He said: "We are making a stand on behalf of all schools and teachers against the irrational, inconsistent and preposterous posturing of Ofsted."
It is claimed inspectors, who visited the 1,000-pupil school for two days in June, regularly interrupted lessons and disrupted graduate teacher-trainees.
Tom Widdows, headteacher, said in one incident an inspector refused to allow a trainee teacher to get a senior colleague's help to deal with an unruly pupil, leaving him intimidated and humiliated.
The academy, which replaced the failing Thamesmead comprehensive two years ago, saw the proportion of pupils with five top GCSEs increase to 36 per cent this year from 6 per cent in 2002.
Ofsted said the "flying" two-day visit was in line with the inspection procedure for schools it was closely monitoring.
A letter was sent to the school in July praising its exam record and noting increased attendance, improved behaviour and a fall in exclusions. But it also outlined concerns over the standard of teaching saying the school had significant weaknesses.
A spokeswoman told The TES it stood by the conclusions but said it was reviewing the findings in light of the school's complaints.