The findings - in the context of a very good report - will fuel controversy about whether the school really needed to ask parents for financial support.
John McIntosh, head of the Catholic school in Fulham, West London, provoked a political storm when he asked Mr Blair and other parents for the payments to avoid a projected pound;250,000 budget shortfall.
However, Office for Standards in Education inspectors, who visited the school in March, found that the deficit ultimately amounted to only pound;91,438.
The inspectors said that "careful management" by the school meant the figure was lower than anticipated. But it was still too high, especially given relatively high funding levels.
"The governing body has a responsibility to provide a budget for expenditure in line with the income it receives," the inspectors said.
Mr McIntos wrote to parents on the eve of the Labour party conference last September, implicitly placing the blame for the expected pound;250,000 shortfall on the abolition of the school's grant-maintained status under New Labour.
Hammersmith and Fulham council said the school received pound;2,994 per secondary pupil this year - pound;300 more than neighbouring Sacred Heart high school, where Mr Blair's daughter is a pupil, but less than four of the borough's eight secondaries.
However, all of these schools have many more pupils receiving free meals, which adds to costs.
The Prime Minister is still not saying whether he has agreed to meet the school's cash request.
The Oratory, a 1,338-pupil voluntary-aided Roman Catholic comprehensive educating boys aged 7-16 and boys and girls in the sixth form, is judged to have many strengths and few weaknesses. Last year, every Year 6 pupil achieved the expected level in key stage 2 tests, while 93 per cent of 16-year-olds registered five or more good GCSE grades.
Mr McIntosh said the report was "first-rate".