Many GCSEs are becoming "insufficiently rigorous", a leading headmistress has said.
A "major re-think" of the qualification's content and structure is needed, according to Bernice McCabe, of the North London Collegiate School.
She suggested that the exams have become too accessible, with too many modules and structured questions.
Her comments came as North London Collegiate, an independent school catering to girls aged four to 18, came second in a new league table of private schools' GCSE results.
Some 97.1% of the school's GCSE and international GCSE (IGCSE) entries were awarded an A* or A grade, with 100% achieving at least a C.
Mrs McCabe said that the school's success is down to a "rich academic life" and focusing on a love of subjects and not teaching to the test.
Mrs McCabe, who sits on the Government's national curriculum review advisory committee, said that her school now takes IGCSEs in a "large number" of subjects, including maths, science, modern foreign languages, computing and music.
"Increasingly, the GCSE is insufficiently rigorous, although it does depend on the subject. There does need to be a major re-think of the content of syllabuses and structure of the questions," she said.
Wycombe Abbey School in Buckinghamshire topped the private schools GCSE league table. In total, 97.91% of its GCSE and IGCSE entries gained an A* or A, and 99.89% scored at least a C.
The Independent Schools Council gathered GCSE and IGCSE results from 564 of its member schools. The overall results showed that 31% of all entries were awarded an A*, down from 31.4% last year, and 60% got at least an A, down from 61.4% last year.
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