Actually, the parsley was meant to be eaten. You'd eat it first if your plate just followed another, or you'd eat it last, if another dish was coming. Parsley is a palate cleanser. It was customary to chew on the leaves, keeping them attached to the stem, and discarding it, discretely, on the edge of your plate.
As for the rest of garnishing, I agree completely with the school that garnishes should be edible, and compliment the flavour of the plate.