Speaking at a London conference on the future of education for 14 to 19-year-olds, Professor Hargreaves suggested the changes as part of an overhaul of the assessment system, which he described as "incoherent and excessive".
He told the conference, organised by the AQA exam board: "The key stage 3 tests could be abolished. I suspect the Government hangs on to them as a measure of the success of the KS3 strategy."
He believes KS3 could be taken in two years, allowing most pupils to sit GCSEs at 15 and then spend three years in the sixth form.
The GCSE would become a two-level qualification. A current grade D to G would give pupils a level one, which Professor Hargreaves, former chief executive of the Qualifications and Assessment Authority, said was usually treated as a failed level two.
The AS-level would become a free-standing qualification, marked by teachers to grades of distinction, merit or pass. Students would only need to pass the subject to go on to A-level, and their AS marks would not contribute to their final A-level results.