Dress Codes and Gender

A top student at a South Carolina high school has been banned from the graduation ceremony unless she wears a dress.

Um, isn't that gender discrimination...?

We had a similar issue at my school a few years ago - one girl, who never wore dresses, fought to wear pants. Our super-strict administration finally gave in, so she wore pants, and a boy in our class wore a dress under his robe. This year, the paper they gave us said that girls could only wear dresses and boys could only wear pants, but I think it's an unstated rule that you can wear something different if you get special permission... or just don't get caught!

Is this reasonable? They want us all to look uniform, and I understand that, but can they do that without enforcing the dress code by gender?