You shouldn’t practice on a full belly, but on the other hand, you shouldn’t practice if you’re hungry either. You don’t want to feel faint from all the forward folds and standing. I would recommend eating a full meal no sooner than an hour before class. If you can’t manage it timing-wise, try to eat a snack beforehand like a piece of fruit, a spoonful of peanut butter, or a handful nuts. You’ll get in the swing of how full you can be during practice once you get in the routine.