Agree completely with Bloss,
as a v. shy child, it wasn't until I went to a school where boys were a small minority (5vs 15 girls), that I really gained confidence and 'blossomed' academically.
Some of you have said about school being more than just about learning things, well, yeas, maybe, but your results at school might affect the rest of your life, if you're in the wrong environment you might get off studying fort he rest of your life and therefore limiting your possibilities right from the start.
As for primary education, I remember at that age I tended to mix with girls only anyway, even if my school was mixed, so it did not make any difference to me having boys at school, apart from the disruption their behaviour caused!
It is also very important to make sure they get enough interaction with boys outside school, of course.