Girl Altar Servers?
I think having only boy altar servers is best. Here's why: boys and girls like to do things separately. When I asked the girls at St Joseph's High School if they wanted to be altar servers they said, "No, father, we don't have to do everything with the boys. We're happy for them to be altar servers and not us." It was their feminist grandmothers who wrote me the letters complaining when we switched to all boy altar servers. Boys and girls like to do things separately because their sexuality (like everything else about them) is in development. Boys doing things with other boys and men help them to become men. Girls doing things with other girls and women help them to become women.
A second reason why I prefer all boy altar servers is that they (and everyone else) associate in their minds that the sanctuary and the altar are the preserve of the priest and deacons, and that the church teaches that this is a masculine role, and therefore reserved to men. Boys serving Mass supports the teaching of the church in this way, and it helps to form vocations early. The feminization of Christianity has been like an unstoppable juggernaut hurtling through our tradition and boy only altar servers helps to put the brakes on that. To speak plainly: religion seems girly enough already, and it's nice to have some roles that only the women do and some that only the men do.
However, that being said, girl altar servers are permitted. In my parish we have girl altar servers,