It's not OK to offend anyone, but for the most part, I think people overblow the significance of this. My son was one of those people who put a Naval Jack on his class ring, and most of the blacks he knew had no problem with it. In Mississippi, a majority of the black population voted to keep the old flag. Whether or not a flag is displayed has little or nothing to do with anything. And I'm not being hypocritical, you're just misunderstanding what I'm saying, as it's complex. But for some reason, I doubt that anyone wears a Confederate flag into a school where a lot of people would be offended, for the same reason as the anti-Southern example, schoolyard justice.
You have to understand, there is a balance to be maintained between being respectful to people, maintaining proper social mores' of conduct, and being able to express yourself however you see fit. It's a hard balance to strike and it's not always a clear line.
Which is exactly the reason this school did what it did. It doesn't want to have clothing problems interferring with the students education. This school decided the balance would be best served by banning the confederate flag. This way, when the time comes for them to ban my suggested t-shirt, or a black panther t-shirt, or a Bush = hitler t-shirt, the school will be doing what it was always trying to do, minimize student conflict.
I don't accept that this issue is complex. It's only complex to those who are absolutely unwilling to place themselves in another person's shoes.