At this rate then my reply is worth 4 cents ...
Those are some good points, Frank. There is a big difference between "freedom of expression" (such as wearing a sexually oriented tshirt or burning the flag) and in "freedom of speech" which is just as you said, the right to *speak* freely. While "freedom of speech" is a constitutional right, "freedom of expression" is not yet, sadly, few people are able to make the distinction.
Evl, as Frank said, there are many good reasons for schools to impose dress codes and limitations as to what sort of tshirts can be worn. The main reason is that certain types of attire are distracting or can be inflamatory. The latter could cause fights between students while the former could cause interruptions to classroom procedures.
Face it ... if the girl sitting just in your line of site is braless and wearing a revealing see-thru top and playing with her bellybutton ring, aren't you going to be paying more attention to her than to the teacher thus being distracted from learning what you are in school to learn?
Tshirts that advocate or advertise drinking, smoking, sexual practices are not appropriate in most schools so I don't see the school did anything wrong in sending her home to change.