I think the young man's best point is that other possibly controversial clothing is allowed. To be fair across the board, I think all messages on school clothing should be prohibited, not just single out the reb flag.
Yep. Can't have that free-speaking going on in education.
The problem with deciding it is okay to limit someone's liberty to do or say what they want is that short of banning everything (e.g. uniforms and nothing else), you end up deciding who you will allow to speak and who you won't. Government becomes the controller of all speach. The first amendment doesn't end with "...as long as nobody is offended."
Frankly, how Anderson County voted on secession is irrelevant to this whole discussion, and a bit of local trivia I would expect no one in HS to know.