Cook shell pasta according to package directions. In a large jar with tight-fitting lid, shake low-fat milk and all-purpose flour until completely mixed. Pour into a nonstick saucepan; add salt and a pinch of nutmeg.
Bring to a boil over medium-high heat, whisking constantly. Continue whisking; cook 3 minutes more. Stir in cheddar and Jarlsberg cheese, and grated Parmesan cheese, whisking until cheeses are melted and well combined. Pour sauce over hot pasta; stir in 1 cup cooked mixed vegetables.
