While you are asking for suggestions re: how your daughter can express herself with the limitations of being uniformed, I'd like to approach this a bit differently. My answer assumes a rigid uniform policy.
I attended a Catholic high school with a very restrictive dress code: one particular blazer (blue for freshmen/sophomores, brown for juniors/seniors) or (bl/br) swearer vest, white dress shirt, (bl/br) plaid skirt, dark blue/brown or white knee socks, brown leather penny loafers. No gaudy jewelry, makeup, headwear, etc. That's it, no exceptions without demerits->detention->worse.
If I recall correctly, though we didn't like the uniforms and would have liked more comfortable clothes, I never worried about self-expression per se through clothes except for the fact that the uniforms flattered no-one.
I suggest you talk to her about self expression and what it means to her specifically. What is she trying to express? Is it a value that is only expressible through the medium of clothing? Or is clothing a medium through which her opinions can be readily ascertainable to strangers? Is it a way of weeding out/warning away people who don't agree with her, or to attract like minded individuals? Is it a way to "fit in" or a way to "stand out"? Is it to appear attractive, and if so, to whom?
In other words, until you (and she) know exactly why self expression through clothing is so important to her that she would trade away a good education for that 'right', you won't make much headway if the dress code is very restrictive.
Once she can articulate honestly the reason she relies on clothing to project what she feels she wants to communicate, you can explore with her other ways it can be done, less easily but perhaps more meaningfully.
If she is a rebel, she can join the school newspaper, radio station, or other after-school activities. If she likes attention, the drama club or glee club might appeal to her. If she wants to change the world, she can learn how to do it from within (i.e. hold office and learn to negotiate), by running for office. Etc. Etc. There are plenty of avenues worth exploring.
Part of a good education is learning to articulate your beliefs in a way that people who feel differently can (hopefully) understand. A more important part - to me - is exploring the foundation of one's beliefs. Talking through her desire to 'dress for the message' is one step in the direction of exploring why she believes what she believes.