For the first layer, combine broken pretzels, melted butter and sugar. Stir together until well blended. Press in 9 x 13 inch baking pan and bake at 350 degrees for 7 minutes. Cool completely.
For the second layer, beat sugar and cream cheese together until smooth. Fold in Cool Whip.
Spread cream mixture over pretzel layer (be sure to seal the cream cheese layer against the sides of the pan so that the jello mixture will not be able to leak through when added).
For the Jell-o layer, mix orange Jell-o and boiling water together until Jello is completely dissolved. Add mandarin oranges and let cool until slightly set.
Pour Jell-o over cream cheese layer and refrigerate until firm.
Cut into squares and serve.
Notes
You can make this pretzel salad a day before or the night before. Make sure to cover it well, and keep it stored in the refrigerator. We have eaten the leftovers (if there are any) for an extra day, and it is delicious. After a few days, the pretzel crust becomes soggy.