In 1992, pupils at fee-paying schools were more than twice as likely (106 per cent) to gain five or more GCSEs at grades A to C than state schoolchildren.
They were also 13 per cent more likely to obtain five or more A to G grades. Two years later, fee-paying students in that year group gained 38 per cent more A-level points on average than state school candidates.
But by 1997 the equivalent gaps were: five or more GCSEs A to C, 93 per cent; five or more GCSEs A to G, 10 per cent; and A-level points, 24 per cent.
The relative improvement in state school scores has been tracked by Dr Stephen Gorard of the University of Wales, Cardiff, who is studying school effectiveness in Wales.
He compared the 1997 statistics with the GCSE year of 1992, because the older children were the last cohort to enter secondary school before the 1988 Education Reform Act took effect.
"In considering the effectiveness of schools since 1988 it is possible to use the fee-paying sector as a control group because they have been relatively unaffected by legislation," Dr Gorard says.
But he adds that there were other indications that the performance of state schools had improved.
"It is possible to predict with 80 to 90 per cent accuracy the GCSE results of each school simply from the socio-economic character of its student intake.
"However, since 1988 the relevance of these background characteristics appears to have declined, meaning that it is possible that the schools themselves are having a greater underlying impact on results."
Dr Gorard suspects that state schools are now "playing the market game successfully" and suggests that some of this change may be linked to increased parental choice.
"Using measures such as the proportion of students eligible for free meals, it has been established that secondary schools in Wales are now more comprehensively mixed in social terms than they were in 1988," he says.
"The relative popularity of fee-paying schools in Wales has declined consistently since 1990 - in other words, state schools are now more popular."
An article by Stephen Gorard, "In defence of local comprehensive schools", appears in the summer 1998 edition of Forum, the journal for comprehensive education.