Boys tend to take longer. They like to play with their friends and form gangs. School is a place where they meet their friends and muck about. Work is a dirty word. The general tendency is to do the minimum work that you can get away with.
Homework is done in a hurry in the minimum time and never checked.
Boys can be good at games and score well in the team. This is a method of improving ones position in the gang pecking order. Girls then do well in their exams throughout school life, usually beating the boys and coming top. If there is a special teacher who gets the boys enthusiastic for that subject, then the boys will be switched on and do well academically in that subject. For the boys it all depends on the relationship with a teacher.