Actually...
The dress code states no clothing or jewelry depicting money ($), playing cards, dice, drugs, tobacco, weapons, gang themes, gangsta, hustler, chiefin' shirts (don't know what da fuck that is), alcohol, sexual contents, innuendo, or reference to any of the above, (and it goes on..).
The shirt in question was a flag using shell casings to depict the American flag along above the words National Rifle Association.
Not sure about you but that's plenty of innuendos and references to weapons. Like many others I've seen kids in my school having to flip their shirt upside down for various reasons (Stone Cold Steve Austin 3:16) was a popular one back in the day. To me this is nothing. If someone can get kicked out of school for a 3:16 shirt, they can get kicked out for a shirt with shell casings...