Ezekiel bread is one exception, but they are also (at my local big box store) 3x the price (by kcal) of the white-label whole wheat bread AND less shelf-stable (thus will incur more process losses and expenses).
That's not nothing of course, especially for a school trying to minimize food costs. Many schools (like the one I attended growing up) procure the cheapest possible USDA/NSLP-approved product.
But kids at these sorts of schools seem like they'd see the greatest benefit from this sort of legislation to raise the threshold of what constitutes a healthy lunch.