Preheat oven to 400. Lightly grease an 8 x 11 x 2 baking dish.
Cook the pasta according to package directions. Drain and set aside.
In a non-stick skillet over medium-high heat, cook bacon until it is browned and the fat is rendered, about 10 minutes. Throw in the white part of the scallions with the bacon for the last 3 minutes of cooking, stirring frequently. Remove from heat and set aside.
Melt butter in a large saucepan over medium-low heat. Whisk in flour until smooth. Continue whisking while gradually adding the milk one cup at a time. Cook for 5 minutes until thickened. Reduce heat to low and stir in salt, black pepper, and most of the cheese, reserving about a cup of cheese. Once the cheese has melted, remove from heat and stir in the bacon and scallion mixture, as well as the green part of the scallions.
Pour the pasta in the greased baking dish. Spoon the cheese sauce over the pasta, stirring to even out the sauce in the pan. Sprinkle the remaining cup of cheese on top, then sprinkle the smoked paprika over the cheese. Bake for 20 minutes until hot and bubbly.