Last year, 42.3 per cent of Year 11 pupils at St John's gained five or more GCSEs with the top A-C grades, comparing favourably with an average of 39.3 per cent in English schools. (These are all notional figures: the real national average is higher.) The average scored by its pupils on the GCSE points system was 34.2, again comparing well with a national average of 33.5.
But in the new assessment, inspectors have taken into account that an impressive 27.2 per cent of pupils' parents have higher educational qualifications, and only 15.5 per cent of children are eligible for free school meals - much lower than at a typical inner-city comprehensive.
Only 2.3 per cent of pupils have statements of special need, and there is also a higher than average number of girls, who tend to do better than boys overall.
And, OFSTED says of its imaginary school, St John's also has only 0.2 per cent of non-white, non-Asian pupils.
The new analysis shows that when compared with schools with similarly fortunate social backgrounds, St John's performs relatively poorly.
Pupils in similar schools score an average of 36.5 points at GCSE compared with 34.2 at St John's.
And over three years, pupils at St John's perform consistently below their social equals, notching up an average of only 33 points compared with 35. 8 in equivalent schools.