When teenagers go out...
there's only so much you can do about their behavior. You can only hope that you've prepared them to face the world and hope they make reasonable decisions. Been there, done that (raised 2 stepsons and got the gray hair to prove it).
Kids will buy what they want to buy. Few of them think of long-term consequences, so they eat the food they like without regard to stuff like fat content. You can lecture them until you're blue in the face about good nutrition, but you won't find too many of them at "Soup and Salad" instead of a pizza place or burger joint.