Tony Blair should be proud to send his children to Hounslow Manor. So why does it face closure?

Take Two comprehensive schools. One is the London Oratory, attended by the Prime Minister's sons, the high-flying haunt of the middle-classes where 93 per cent of the pupils achieve five good grades at GCSE. The other is Hounslow Manor in west London, where just 17 per cent of the pupils achieve five good grades, where more than one-third are poor enough to qualify for free school meals, and which, if the Prime Minister has his way, could be under threat of closure in the next few years.

Take Two comprehensive schools. One is the London Oratory, attended by the Prime Minister's sons, the high-flying haunt of the middle-classes where 93 per cent of the pupils achieve five good grades at GCSE. The other is Hounslow Manor in west London, where just 17 per cent of the pupils achieve five good grades, where more than one-third are poor enough to qualify for free school meals, and which, if the Prime Minister has his way, could be under threat of closure in the next few years.

Yet, inspectors from Ofsted, the education standards watchdog, have just revealed some startling facts. When they visited the Oratory they judged that 17 per cent of the teaching they saw was very good. None of it was excellent. When they visited Hounslow Manor they decided that 23 per cent of the teaching was very good and2 per cent was excellent.

Under a new policy announced earlier this year by David Blunkett, the Secretary of State for Education, schools must ensure that 15 per cent of pupils achieve five or more GCSE grades at A* to C over the next three years, irrespective of the pupils' background - or they will face closure and a "Fresh Start" under new management.

Roger Shortt, Hounslow Manor's head, says that his school is in danger. Because the turnover of pupils is so high and so many of them speak English as a second language, he fears that this year's GCSE results will slip below the magic 15 per cent figure. Last year, they went down from 21 per cent to 17 per cent.

If Hounslow Manor were to close, the inspection report makes clear, a school which is just as good as the one attended by the Prime Minister's sons would disappear.

The same team of inspectors went to both the Oratory and Hounslow Manor. They pronounced both good and the comments in both reports on teaching, learning and leadership are remarkably similar. At both, inspectors say teaching is "consistently good" - at Hounslow Manor, nearly two-thirds of lessons were thought good or better; at the Oratory, it was just over two-thirds. At the former, there is a "team of dedicated, flexible and innovative teachers; at the latter, "teachers know their pupils well and are always willing to help individuals".

Mr Shortt provides "very good leadership"; the Oratory's head, John McIntosh, gives "decisive and effective leadership".

For the rest, the inspectors say, both schools have their weaknesses. Hounslow needs to improve performance at A-level; the Oratory must do better in its relationship with parents. The former should improve its personal, social and health education; the latter is not complying with national curriculum requirements for design and technology.

So what is the difference? Why do so many pupils at the Oratory achieve well at GCSE while so few do so at Hounslow Manor? The answer is in the first paragraph of the reports - the pupils. Pupils at the Oratory usually stay there throughout their school career. Those at Hounslow Manor come and go with bewildering speed: two-thirds of the 16-year-olds have joined the school after the age of 11. "Approximately, half the pupils have English as an additional language, but 150 pupils, a very high number, are at the early stages of learning English." At the Oratory, only nine pupils are in the early stages of learning English. Only 7.9 per cent of pupils at the Oratory are eligible for free school meals - the indicator commonly used as a measure of social deprivation - compared with 35 per cent at Hounslow.

Pupils' attainment when they enter the Oratory is well above the national average. At Hounslow, it is well below and nearly one-third of pupils have special educational needs.

Mr Shortt said the insistence that all schools should be asked to get 15 per cent of pupils through five good grades was "nonsense. We have an Ofsted report which proves it. If a qualified Ofsted team comes and says a school is doing a good job, that should be it. People need to go to a school and experience its quality rather than making judgements on the basis of statistics."

He also questioned the role of exam league tables which offer only raw results, do not take into account pupils' backgrounds and fail to measure the progress pupils have made.

"The local community see us as a place with a lot of refugees and they look at the league tables and say that is not the place for my child. But we have a very rich and absolutely wonderful school."

The Government's announcement this week that pupils from overseas whose first language is not English who have arrived during the 21 months before GCSE exams will not be counted in the league tables will help a little.

Mr Shortt said: "Of course, we are pleased, but it doesn't take into account other pupils who arrive in the school less than two years before GCSE. It also assumes that younger pupils who join us with no English are not at a disadvantage."

Doug McAvoy, the general secretary of the National Union of Teachers, said: "This illustrates the irrelevance of league tables and the inappropriateness of nationally dictated targets. The achievement of those targets is inevitably much more difficult in one school than in another where circumstances are dramatically different. Failure to achieve those targets is not a reflection of poor teaching."

Mr Blunkett promised before the last election that Labour would introduce "value-added" league tables which will show the progress pupils have made.

Headteachers recently attacked the Department for Education for its delay in producing performance tables which would give a fairer picture of schools. Officials said that the process was complicated and that they were determined to get it right. They promised improved tables within the next few years.

John Dunford, the general secretary of the Secondary Heads Association, said: "The comparison between the two inspection reports illustrates that the Government's policy requiring schools to get at least 15 per cent of good grades is a nonsense. It also reveals how that policy threatens to undermine Ofsted's judgements."

Mr McIntosh was not available for comment.