The Manchester University study of 1,500 teenagers found they had mixed reactions to being split up. Boys feared they would be unable to control their behaviour. One said: "I think it will be a disaster. It's bad enough in PE."
They said girls helped them with their work and that in single-sex classes the teacher pent more time shouting at misbehaving pupils.
While a number of girls missed "the class clown", many felt they got more work done without the boys messing around and "taking the mickey" all the time.
Pupils felt they worked harder in single-sex classes and teachers said pupils were more "relaxed". The Government has funded a three-year research programme into the causes of and solutions to boys' underachievement.