That's what makes me suspect that this isn't part of a principled stand to ban all possibly divisive symbols but rather just a stereotypical cave in to the current PC.
There had been a cross burning on a black family’s (teacher??) yard a couple years ago or so. The rule to ban the Confederate flag came about because of that incident I believe.
The school has had other rules in place for awhile regarding wearing bandanas (gang colors I suppose) and other symbols that “interfere” with safety and learning. But, acording to the kids suspended over the flag, those rules weren’t really enforced. (Perhaps a polite “put your bandanna in your backpack please”).