Position an oven rack in the center of the oven and preheat to 400 degrees F.
Bring a large pot of salted water to a boil. Cook the pasta shells until just slightly softened but still firm, about 7 minutes. Drain well, then rinse under cold water. Arrange the shells in a single layer on a baking sheet, without touching each other.
Cook the chicken tenders according to the package directions. Slice the tenders into quarters; set aside.
Meanwhile, stir together the ricotta, Parmesan, Italian seasoning, 1 1/2 cups of the mozzarella and 1/2 teaspoon salt in a medium bowl. Transfer the mixture to a medium resealable plastic bag. Snip 1/2 inch off one corner of the bag.
Spread 1 cup of the marinara sauce on the bottom of a 9-by-13-inch baking dish. While holding a shell open with your fingers, pipe full with the ricotta mixture (about 1 1/2 tablespoons), then transfer to the baking dish. Press 1 chicken strip on top of the ricotta mixture. Repeat with the remaining shells, ricotta and chicken. Spoon the remaining 1 1/2 cups marinara sauce over the shells, then sprinkle with the remaining 1 1/2 cups mozzarella.
Bake, uncovered, until the cheese is melted and golden brown and the casserole is bubbling slightly on the sides, about 25 minutes.
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