Exam chiefs have hit back at a call to scrap a scheme which gives thousands of Scottish pupils higher grades than their exams marks merit.
The scheme is said to benefit private schools
Concerns were voiced that the derived grades procedure favours schools from richer areas.
The Scottish National Party raised questions about the scheme at a meeting with the Scottish Qualifications Authority on Tuesday.
But the SQA denied it was open to abuse by teachers over-estimating results.
The derived grades procedure gives upgrades to pupils who miss out on the grades predicted by their teachers.
TOP 20 SCHOOLS FOR DERIVED GRADES
George Watson's College 67
Hutchesons' Grammar 59
Williamwood High School 56
Madras College 55
St Ninian's High School 55
Dollar Academy 42
Stewart's Melville College 42
St Aloysius' College 40
Cults Academy 38
St Brides High School 36
Linlithgow Academy 34
High School of Glasgow 34
Bearsden Academy 30
Eastwood High School 30
Lenzie Academy 30
Balwearie High School 29
Banchory Academy 29
George Heriot's School 29
Largs Academy 27
Larbert High School 26
A pupil's exam performance is usually compared to the result expected by the school, then a computer compares that prediction with the accuracy of the school's previous estimates.
The checks can mean grades being improved - or failures converted to passes - before the results are issued.
They can also highlight problems with marking.
About 95,000 different elements were examined last year.
As a result, between 5,000 and 9,000 pupils had their grades improved but campaigners claimed that those at school in poorer areas were still missing out.
The average school got seven derived grades at Higher level in 2004.
Five out of the 10 schools with the most derived grades were private. George Watson's College in Edinburgh got the most with 67 and Hutchesons' Grammar School in Glasgow was given 59.
Two comprehensive schools from East Renfrewshire also did well, with 56 at Williamwood High School in Clarkston and 53 at St Ninian's in Giffnock.
Teacher John Milligan, from Sutherland, has set up an internet campaign aiming to get rid of derived grades.
He said it was already hard enough for pupils from poorer areas to overcome the hurdles in their path.
SNP education spokesperson Fiona Hyslop said: "A derived grade is basically a safety net, but it benefits the rich to the detriment of the poor.
"You have more chance of performing better in a better-performing school from a more affluent area than you have if you are one individual battling against the odds.
"Fairness and equality is a foundation of Scottish education. That is why derived grades are so unfair and the SQA has to sort it."
Fiona Hyslop has called for action on the issue
However, SQA director of operations Tom Drake said the organisation did not have any doubts about the process.
"We have monitored it carefully over the years and we have carried out research that indicates that it does what it is supposed to do.
"For a teacher to try to subvert it would be to risk the performance of the bulk of their candidates, so it would be a very foolish thing for them to do," he said.
However, BBC Scotland has seen an SQA report suggesting there is some evidence that teachers could be over-estimating what their pupils will achieve.
As a result, the authority will start random spot checks of the classwork of pupils who get derived grades.
But that is not enough for campaigners who maintain that the whole system is flawed and should be scrapped.