"Perhaps the way for the school district to address this issue is to have a policy that forbids printed T-shirts altogether "
Now there's a thought. That would be my call. If they allow printed T-shirts, they're going to have to allow any that aren't obscene. That's how free speech works. If they allow no printed T-shirts at all, then there it is. No printed T-shirts. That's equality under the law.
Better to go to school uniforms, save money by only needing one or two outfits for school. Maybe bring back "Yes Sir" and "Yes Ma'am"