Cook macaroni in salted water according to package directions. Pour into colander and rinse well; drain.

In a saucepan, melt 2 tablespoons margarine. Blend in flour, salt and dry mustard. Add milk and heat over medium heat, stirring constantly, until sauce thickens and is smooth (if sauce gets lumpy, stir with a wire whisk). Add 1½ cups cheese; heat until cheese is melted, stirring occasionally. Add macaroni and stir until well blended. Pour into a 2-quart greased casserole; top with cheese and croutons; dot with remaining 2 tablespoons margarine. May be prepared in advance to this point and refrigerated.

Bake at 375° F for 30 minutes or until nicely browned and bubbly. Yield 4 to 6 servings.