Your 2nd and 3rd reasons are almost the same, you need to mold them together. So really, you only have 1 good reason, that they need to learn how do deal with the other gender because it is most like the real world.

Another reason I would put down is diversity – males and females can have different ideas, come from different walks of life, and view life differently. In groups, a female may provide insight that a male just would not think of.

Another reason I would put down is social groups. Males and females usually separate themselves in groups, and having that male-female stigma creates a sense of order and place within the schoolyard. It also may help build comraderie in each of the groups as well.