Preheat the oven to 350 degrees F. Cook pasta according to package directions; drain and return to pot.

Meanwhile, in a large screw-top jar, combine 1 cup of the milk, flour, mustard, salt and pepper; cover and shake until combined. Pour into a medium saucepan. Stir in remaining 1 cup milk. Cook and stir over medium heat just until bubbly. Reduce heat to low. Add cheddar cheese and American cheese, stirring until melted. Pour sauce over pasta, stirring until combined. Spoon into a 1-1/2-quart casserole.

Bake, covered, for 15 minutes. In a small bowl, stir together croutons and Parmesan cheese; sprinkle over casserole. Bake, uncovered, about 5 minutes more, or until heated through. Let stand 5 minutes before serving.