In a mixing bowl, combine the instant vanilla pudding mix with the 6 cups of milk. Whisk together until the mixture thickens, about 2 minutes.
In your 9x13 inch baking dish, spread a layer of the prepared pudding mixture at the bottom.
Arrange a layer of graham crackers on top of the pudding. Break them if necessary to fit snugly.
Continue alternating layers of pudding and graham crackers until you run out of ingredients, finishing with pudding on top.
Spread the Cool Whip evenly over the top layer of pudding.
Cover with plastic wrap and chill in the refrigerator for at least 4 hours, preferably overnight.
Slice into squares and serve chilled.
