Combine sugar and flour in a saucepan. Stir in milk mixture gradually. Place saucepan over medium heat; cook and stir mixture is the consistency of pudding, about 10 minutes.
Arrange an even layer of wafers in the bottom of a large bowl. Top with a layer of banana slices. Cover with a layer of pudding. Repeat layers with remaining wafers, banana slices, and pudding, ending with wafers.