Preheat the oven to 375°F (190°C).
In a large mixing bowl, combine the uncooked penne pasta, Italian seasoning, garlic powder, salt, and black pepper. Mix well.
Spread the uncooked pasta mixture evenly in the bottom of a 9x13-inch baking dish.
Place the frozen meatballs on top of the pasta.
Pour the marinara sauce over the meatballs and pasta, making sure it covers everything evenly.
Add 2 cups of water to the dish, gently stirring to combine without disturbing the meatballs too much.
Sprinkle the shredded mozzarella cheese on top of the casserole.
Cover the baking dish with aluminum foil (if desired, to prevent over-browning).
Bake in the preheated oven for 30 minutes.
Remove the foil and bake for an additional 15 minutes, or until the cheese is melted and bubbly, and the pasta is cooked through.
Remove from the oven and let it cool for a few minutes before serving.
Garnish with fresh parsley if desired, and serve warm.