Secondly, students do not leave their 1A rights at the schoolhouse door: Tinker v. Des Moines (1969)
Yes and no. Students most certainly do leave some of their 1A rights at the door. Tinker v. Des Moines was not carte blanche stating schools had no control over any aspect of the 1A. Schools can and do have dress codes, can and do tell students when and where they are allowed to voice themselves and in what manners. Remember that Tinker v. Des Moines allows for free speech so long as it is not disruptive or interfering with the educational process. This is a key caveat.
So the question then is whether or not the shirt can be considered disruptive or not. Obviously, nobody here will think so as we all support the topic.