Dress codes sure are a touchy subject, aren't they? On one hand, your teachers and school administrators have the right to make sure that school buildings are free of distractions so that you are in a good environment for learning. But on the other hand, clothing is a great outlet for expressing individuality, and limitations on personal expression should always be carefully considered.
To answer your specific question, I'm no attorney, but I don't think a dress code violates your First Amendment rights unless it prohibits you from wearing something you need for religious or health reasons. Some items on school dress codes are simply not negotiable. Colors and clothing that can be clearly associated with gangs or obscene, vulgar messages shouldn't be tolerated. Political speech or clothing that condemns someone else's beliefs should be regarded carefully, too — at worst, it could incite violence; at best, it's likely to distract other students from their studies.
While it might seem like a good idea to defend your rights, consider this: What feels like a big deal now is something you'll probably have to tolerate later in life. Wherever you go in the working world, you're almost certainly going to find some kind of dress code. An employee of a restaurant would be sent home if he arrived at work in the wrong uniform. And you've got to admit, you're pretty unlikely to see one of your bank tellers working in sweatpants or a teacher in front of the class wearing cutoff shorts.
All this aside, I don't think that students should lose all their rights as soon as they set foot on school property. You do — absolutely — have the right to ask for a dress code that's clear, understandable, and universally enforced. If one teacher considers an article of clothing unacceptable that another teacher doesn't think twice about, by all means go to your school administration and point out the inconsistency. Remind them that subjective rules often do more damage than good and suggest ways to improve the dress code. Make sure they understand that all students should be treated equally and all teachers should apply the same reasoning.