[U]nfortunately, I think it's going to be tough to fit this into even the limited rights students enjoy.
If by 'this' you mean the T-shirts described in the article, I think you're probably right. The kids might have an equal-protection argument, but it probably won't trump the school officials' decision.
It's too bad; my personal preference would be to allow all the shirts (and welcome to American public discourse, kids).
...but I would certainly draw the line in such a way that this particular shirt, and expressions of opposing views, were permissible. It seems to me that these are students on the verge of joining the adult world - allowing them to gain an adult understanding of public discourse is certainly a worthy educational goal.