Experts believe that the decline in girls' performance may be due to a collapse of confidence after the age of 16.
A-level exams and courses, they argue, may also be less suited to girls who are hard-working but less inclined than boys to take risks.
Latest official statistics from the Department for Education show that girls achieve lower A-level grades than boys with the equivalent score at GCSE.
But the brightest girls seem to fall further behind than their less able peers. More girls than boys are passing A-level but fewer get higher grades. Among candidates with an average score per subject of six - a grade B - at GCSE, 23 per cent of boys but only 16 per cent of girls get three As at A-level.
Jannette Elwood of London University's Institute of Education, who has researched why boys outperform girls in the top grades at A-level, concluded that boys' risk-taking approach may be better suited to A-level. At GCSE girls' hard-work and consistency pays off.
According to the teachers questioned in the survey, boys' faith in their ability may override their lack of knowledge and skill. Ms Elwood said: "I think there is a lack of confidence once girls get to A-level. It may be something to do with meeting a different way of working.
"The way you engage the examiner is different: getting to the point, sparkle, is valued and boys tend to show that more than girls."
If examiners were looking for different styles at A-level, pupils and their teachers should be told so that they could prepare.
Madeleine Arnot of Cambridge University's department of education, co- author of an Office for Standards in Education report on gender and educational performance, said: "Boys seem to make greater progress between GCSE and A-level. There is more work to be done to bring the rates of progress of boys and girls together.
"Girls are obviously perfectly able to perform well, so this raises questions about their experience and whether they are sustaining their levels of confidence.
"Is stereotyping more of a problem between the ages of 16 and 18 and are girls getting the support and encouragement they need?"
She argued that girls might not have acquired the deeper confidence they needed before the age of 16. "There is a difference between short-term confidence and long-term confidence."