1) Pupils generally should behave well. When they are quiet and respectful, teachers don't have to make them shut up and listen. So teachers can impart knowledge, pupils can focus on gaining this knowledge. Although in reality there are always a few pupils in the class who misbehave. Some of them want to be the center of attention. Some want more control in the classroom.
2) There are people who believe that boys' and girls' brains function in different ways, so they learn very differently. Therefore, if they are taught separately, their teachers can use specific teaching methods which leads to girls' better academic results. Besides, girls receive better marks at school than boys because they are better behaved. Girls are also more hard-working.