Of course I largely agree with you. I've never known it any other way than in mixed classes. Nevertheless, educational researchers, who I consider to be very wise and progressive, recommend separate lessons in certain areas: simply because the mechanisms by which boys and girls grasp the same topic or problem are fundamentally different. For example, boys learn a lot by trying things out and girls learn by imitating. It's a broad field and I guess ideally every child would be encouraged and educated individually...