Combine the first 6 (dry) ingredients in a mixing bowl. Make a well in the center of the dry ingredients and pour in the yogurt and oil. Stir together until smoothly combined. Stir in the zucchini, raisins, and optional walnuts.
Divide the batter between 12 muffin tins and bake for 20 to 25 minutes, or until the tops are golden and a small knife inserted in the center of a muffin tests clean. Cool on a rack. Wrap well in plastic wrap to send in a lunch box.