Preheat oven to 350 degrees F (175 degrees C). Lightly grease and flour a medium baking dish.
Place squash in a pot with enough water to cover. Bring to a boil and cook 5 minutes, or until tender; drain.
In a bowl, mix cooked squash, cornbread, onion, celery, sugar, egg, soup, margarine, and 1/2 cup cheese. Season with sage, salt, and pepper. Transfer to the prepared baking dish.
Bake 45 minutes in the preheated oven. Top with remaining cheese, and continue cooking 15 minutes, or until cheese is melted and bubbly.
