In a stockpot on medium-high heat, break apart and cook ground beef and onions until no longer pink. Drain off excess grease.
To the pot, add the chili beans, tomato purée, chili powder, salt and water (start with 2 cups and add more later as desired). Bring to a boil, then add sugar and spaghetti. Cook until the spaghetti is al dente. Taste and add more salt and water if desired. Note: I added 1 more cup of water to equal 3 total.
Place 2 sheet pans in an oven preheating to 425°F.
Spread a thin layer of butter to the edges of one side of each slice of bread. Once the pans are heated, place half of the bread slices buttered side down on one of the baking sheets. Top with cheese slices, then with another slice of bread, buttered side up. Place the other sheet pan directly on top of the sandwiches.
Bake 8-10 minutes then broil with the top pan off until the bread is toasted.
To create that chili lunch school memory, complete the lunch tray with a bowl of chili, crackers, grilled cheese, carrots and celery sticks, a no-bake cookie and milk.