Was I wrong?

I think you were a little heavy-handed but overall I agree with you. I think secession was probably legal, and the invasion of the Confederacy was probably unconstitutional, but the outcome was certainly the right one, even without justification. It's worth a few talking points, and it could be a great opportunity for you to teach your teenager something about history that he isn't going to get in school. But I'd probably have freaked, too, if one of mine had worn that shirt.