Almost a third of GCSE entries from private school pupils were awarded the top grade this year, figures show.
New data also reveals a continuing trend of fee-paying schools choosing international GCSEs (IGCSE) over the traditional exams.
Some independent school leaders said they believed the IGCSE was tougher and better suited to pupils' needs.
Figures published by the Independent Schools Council (ISC) for 560 of their member schools show that 32.9% of exam entries - GCSE and IGCSE combined - were awarded an A* grade this year, up marginally from 32.7% last year.
This is around three times as many as the national average.
Overall, a typical teenager attending a private school scored the equivalent of two A* grades and seven As, the ISC calculated.
The figures also show that 446 of the council's member schools had pupils taking at least one IGCSE this year, while three only took these qualifications.
In total, around four in 10 exam entries (43.3%) were for IGCSE this year. This is up 12.3% on last year.
In 2010, just 11.1% of exam entries for Year 11 pupils were for IGCSE, the ISC said.
ISC chairman Barnaby Lenon said that the qualifications are continuing to grow in popularity.
"Independent schools value the freedom to offer both the IGCSE and the GCSE as they think best for their pupils," he said.
John Claughton, chief master at King Edward's School in Birmingham - where 88% of all entries were awarded at least an A - said almost all of the exams taken by their pupils were IGCSEs.
"We believe the content of the IGCSE is more challenging and is much better preparation for the IB Diploma taken by all our boys. IGCSE does not waste time on coursework and controlled assessment tasks which consume valuable teaching time.
"Our boys have done better in IGCSE in recent times precisely because it has asked harder questions and rewards understanding."
Paul Baker, deputy head for curriculum at The Perse School in Cambridge, which saw 94% of all entries graded A* or A, said: "The IGCSEs that some departments have chosen suit our students' progression needs better.
"For example, the mathematics IGCSE contains more introductory calculus and work on functions which prepares students well for A-level."
From this September, new GCSEs in English and maths will begin to be taught in schools and colleges across England, with further subjects introduced over the next two years. The new courses are being brought in following a major shake-up of the exams system under the coalition government, designed to make the qualifications more rigorous.