The shirts are a violation of the dress code. The dress codes states "Any apparel with pictures or writing that is vulgar, obscene, distasteful; that advertises alcohol, drugs, or tobacco; that is sexual in connotation, promotes nudity, or implies any of the aforementioned."
It is my opinion, and the opinion of other teachers and [the high school principal], that this shirt implies partying--in the "real" sense of the term--which is not appropriate for school. I have no doubt that many of the kids just bought the shirts because their classmates did, and the reference to partying carries no weight. But for some of the students in the class, this reference to partying is all about alcohol and possibly drugs. We have to draw the line somewhere, and if we begin to allow these things to come into the school, we will soon have a nightmare on our hands in trying to enforce the dress code.
Because of that, I have asked the students that I see to take the shirt off or turn it inside out. In my opinion, this wasn't exactly the best shirt for the class to buy. There are many other things they could have put on their shirts that would have been much more appropriate for school, but they chose to reference partying.
If you have any further questions, please do not hesitate to ask. Thank you for your concern.