I used to make this all the time and it's a fun cake to make.
Elementary kids love making this. There's some magic involved during the baking process and if the kids are the ones doing the pre-baking assembly, they'll be real excited when the cake comes out of the oven.
1 (10 ounce) bag miniature marshmallows
prepared yellow cake mix
1 (3 ounce) box strawberry jello
1 (16 ounce) box frozen sliced strawberries
2/3 cup cold water
Cool whip, optional
Spread miniature marshmallows in single layer to cover the bottom of a 9 x 13" greased baking pan. Next spread prepared yellow cake mix over that. Sprinkle strawberry Jello powder over wet cake batter. Then drop by spoonful, frozen sliced strawberries, slightly thawed. Drizzle cold water over all without stirring. Bake at 350F for 60 minutes. Let cake cool for 30 minutes on rack before cutting. Serve with Cool Whip if desired.