The expert - university professor Caroline Gipps - says boys get along better at single-sex schools.
"They are likely to be less anxious about working hard and asking questions because this is not seen as being done in front of girls," she said.
Hardly any lads in mixed comprehensives go on to study the subject at A-level because they don't like asking questions in front of their female classmates.
This year's exam results showed that boys made up less than a third of all the pupils who took English A-level.
Most of these came from private, grammar or same-sex schools.
A lack of male teachers taking the subject is also being blamed for the situation because kids often look up to their tutors as role models.