Like it or not, the full rights granted are not complete until you turn 18. Therefore, schools do have a right to create their own environment, their own code of conduct. However, you have a right to be aware of these rules.

While I do think that a five day suspension was a bit harsh, I also do not know the details of what happened. Did you mouth back? Did you refuse to take off the t-shirt? Those things (and more) could have contributed to your suspension.

Finally, the reason they ask students to take off distracting shirts is because they want schools to be a place where everyone feels as comfortable as possible. Granted, that is in an Utopian society, but school officials have the right to try to guarantee a student's right to learn in a comfortable environment. That being said, try discussing your shirt and your concerns with your school officials. They actually do listen from time to time.