Heat oven to 350*F. Coat an 11x7x2-inch baking pan with nonstick cooking spray. Bring large pot of lightly salted water to a boil. Add rotini and cook 5 minutes. Add scallions and cook an additional 2 minutes. Drain and set aside.
In a medium-sized saucepan, melt butter over medium heat. Whisk in flour until smooth. Gradually add milk, whisking constantly, until smooth. Stir in salt and pepper; bring to a simmer. Cook, simmering 4 minutes. Remove from heat.
Add half of the shredded cheese to milk mixture; whisk until smooth. Stir in corn, then combine in large bowl with pasta and scallions. Pour half into prepared dish; top with half of remaining cheese. Repeat, ending with cheese.
Bake at 350*F for 25 minutes. Cool slightly before serving.