"So you have to wear a plain t-shirt? I can understand not wanting halter tops and tank tops and such, but what is wrong with graphic T's?"
Uh, it may have something to do with what those graphics say or imply.
Not all graphic Ts say or imply bad things. Granted schools have slightly different rules to live by regarding speech, however, this does not seem to meet the restrictions of time place and manner laws. It is certainly not narrowly tailored to restrict the minimum amount of speech.