[quote=The Nexerus]
Dress codes exist because if they didn't, people would wear things to school that wouldn't have been allowed under a dress code.
[/quote]

...Well, obviously.

I don't understand why that means that students aren't allowed to wear leggings, though. 

Basically the only thing this post told me was "a rule exists because if it didn't exist people would do things beyond the limitations of said rule because, you know, the rule wouldn't exist". And that's kind of true for every sort of rule, at least in theory. That still doesn't explain the reasoning behind said particular rule. I mean, I can understand loose dress codes barring anything excessively "unreasonable", but leggings? This, your post does not answer.