Ladies and Gentlemen!

At my school, we have a dress code. The men must wear dress slacks, dress shirts, and ties, and the ladies must wear blouses with skirts that fall below the knee. Whenever I talk about my school with someone, they hear the word "Catholic" and almost inevitably ask if we have a uniform. And I explain the dress code. "What? You mean...you never get to wear pants? Gosh, I could never do that." "Wow, how can you stand it?" "That'd be so uncomfortable. Even in the winter?" "Why can't you wear pants? Do you think there's something wrong with that?"
Some people think that it takes away some of the freedom that women have worked so hard for. Women worked for so long to be able to vote, be payed fairly, and....wear pants! Is the school trying to take that new freedom away from its girls?

So, why do we wear skirts?

It's true that women can look modest and professional and even pretty in pants. Many women do. But I think that we lose something beautiful when we lose our long skirts and dress more like men.

Being a woman is different from being a man, and it is good! It is in no way less, and it is very beautiful.
"I do not understand why some people are saying that women and men are exactly the same, and are denying the beautiful differences between men and women. All God's gifts are good, but they are not all the same.
"What a woman can give, no man can give. That is why God has created them separately...Woman is created to be the heart of the family, the heart of love. If we miss that, we miss everything. They give that love in the family or they give it in service, that is what their creation is for." (Mother Teresa)
"Man and woman have been created, that is to say, willed by God: on the one hand in perfect equality as human persons; on the other, in their respective beings as man and woman. [...] But the respective "perfections" of man and woman reflect something of the infinite perfection of God: those of a mother and those of a father and a husband." (CCC, 369, 370)

Human sexuality is different from that of animals; it affects us body and soul. It's not just external. But it's not just internal, either! God created man and woman with different kinds of souls, and with different bodies which reflect the differences in the soul.
People have a real need for external signs. We naturally learn through our senses. We learn what things are by seeing, feeling, smelling, hearing, tasting them. We discover substance by means of accidents. ("Accidents" here doesn't mean car crashes, it means something's attributes.)
God knows this. He created us with this dependance on physical things, with this need for signs, on purpose. And the understanding and compassion that He shows us in this way is great. "And the Word became flesh, and dwelt among us." He gives us external signs of internal realities supernaturally in the Sacraments, and also naturally, in the things of the world!

So God has given us signs, and he provides for our nature and our need for signs. But because man is a rational animal, He left us to do some things on our own. He gave all the other animals fur, for instance, to protect them and make them look beautiful (imagine a bald dog or a bear...gross!). They are born with clothing, but we are not, because we can make it. Clothing is a certain perfection of man. Because men and women are different, and our bodies reflect those differences, it makes sense that
our clothing also should.

Look at the differences between men's and women's bodies. In Shakespeare's The Taming of the Shrew, Kate says (Act V, Scene II),

"Why are our bodies soft and weak and smooth,

Unapt to toil and trouble in the world,

But that our soft conditions and our hearts 

Should well agree with our external parts?"

("Conditions" means something like "qualities of behavior".)

Men's bodies are suited to physical labor, and women's bodies aren't. So it would make sense for men to wear clothes that are better for that. Even when a man isn't actually working or fixing cars and moving bookshelves, he is still a father and a husband; he is the protector and laborer of the family, his wife's and children's defender, and his clothes should be a sign of that. (Sorry, Scotland! ;-) ) Also, we seem to cover up things we want to protect. Men's sexual organs are even on the outside, but women's are on the inside, veiled. Clothes could reflect that, too. Even the word "woman" means "veiled man." (Of course, moderation is always necessary...I'm not saying we should cover ourselves up entirely!)

But above all, skirts are just pretty. They really are. Clothes are supposed to make you look beautiful! That's mostly why we wear them. I'm not saying that women can't look nice in pants, but if you take any woman and put her in a nice dress (that fits!) and then put her in a nice pair of pants, the dress will always look better. Guaranteed.