HEAT oven to 350°F.
COOK macaroni as directed on package. Meanwhile, melt butter in large saucepan on low heat. Stir in flour; cook 2 min. or until bubbly. Gradually stir in milk; cook on medium heat until mixture comes to boil, stirring constantly. Simmer on low heat 3 to 5 min. or until thickened. Add 1-1/2 cups cheese; cook and stir 5 min. or until melted.
DRAIN macaroni. Add to sauce with bacon; mix lightly. Spoon into 1-1/2-qt. casserole; top with remaining cheese.
BAKE 20 min. or until heated through.