Addressing a London conference to mark the curriculum's 20th anniversary, he said the longer day could be a reality if more support staff were recruited to reduce the amount of time teachers spend marking and preparing work.
He said: "I regret that in 1989 I did not extend the teaching day by one period to accommodate a larger curriculum - accommodated, of course, by more staff to reduce the administrative burden on teachers."
This was impossible, he said, because it would have meant renegotiating an agreement with the unions which had settled the 1980s teachers' strikes.
The curriculum has been criticised by teachers and academics for being overloaded since its introduction. Lord Baker said that city technology colleges had benefited from the freedom to lengthen the school day, as had academies.
He also told the conference at London University's Institute of Education that "draconian" measures should be introduced for primary children who misbehaved. These would include sending them to boarding school behaviour referral units for two to three years.
The discipline problems facing schools, he said, had worsened dramatically since 1988. He said: "A generation of pupils in our schools have not learnt the word 'no' from their parents. The strain on teachers in dealing with pupils who will not obey must be astronomical."
Lord Baker welcomed the Government's new diploma qualifications, but said their benefits risked being lost because pupils would have to travel between schools and colleges to take them. He proposed re-introducing technical schools for 14- to 19-year-olds to offer the diplomas on one site. Pupils would remain in primary education until 13 or 14, he added.
He said the idea was being taken seriously by the Government.