Here's the good things:

First of all, you set a standard to maintain. That's great. The child has concrete and clear lines of what is expected of her. Straying beyond these requirements is a sign that she does not trust your wisdom. That is not an excuse to act out beyond the requirements though.

Secondly, you enforced the standard. You told her what is expected, she didn't follow it, thus the consequences were brought forth. Perhaps your methodology could be perceived as "brazen and hasty" by some, but in the end, you have the final say. You're the parents.

Here's the bad things:

We've all heard the classic "if they all jumped off a bridge, would you do it too?" The articles of clothing her friends' parents allow are non of yours' or your daughter's concern. You are the provider for her. She is dependent upon you for survival until, at least, to the age of 18 (if you are States dwellers). The "my house, my rules" still applies unless she would prefer to chip in on room and board.

While making her wear ankle length skirts could make her a social pariah, perhaps she should have listened to your guidance from the very beginning. I was friends with a girl in high school who wore ankle-length denim or cotton skirts every single day. She was one of the most popular girls in school because she didn't draw attention to the fact that she had to wear those items. If she would stop making a big deal of it, lessthe attention would be drawn away from it, and she may have a better time.

Also, I don't think length of skirt is the issue here. She's seeking to test you; to test what measures you will take to ensure enforcement. Don't give in. Stay steadfast in your guidance. She may not like it now, hell, she may not like it in the future, but at least you'll show her that people should expect something of her rather than just giving her the world at the first complaint.