That's because if we study with a single gender, let's say being a girl I study with girls and am fond of them. I gain experience with girs only. But when I grow up have a job or am in a university, where there are lots of boys I need to work with and talk to on daily basis, what will I do? I'll be just like a fish out of water. I'd be shy talking to them because for all my life I talked and did assignments with girls. I won't be that much easy or comfortable with them. I may feel awkward at times. To prevent this it is neccesary to study in co education schools from the start. Even if there is religious prohibition, for example in Islam girls are not allowed to touch or even come in front of boys they don't have blood relation with. Without purdah. So being muslims (or if there is another religion with the same prohibition) you can take care of that. You can wear full sleeves and cover your head with a scarf. I am a muslim and I do that. Now atleast when I grow up I won't lack confidence if I talk in front of boys.