Research has shown that teenagers may be able to predict the future 2 days in advance.
This is one of the sticking points for my students, who really only think day to day, and sometimes just in the moment. Maybe as far as lunch, because it's a great "reward" for studying all morning.
More one to one discussion would help tease out what each student wants to do in the future and why they would want to save for it. That's accompanied by breaking it down monthly and weekly. Once they have that in mind, they would be more successful in accepting ideas for saving on a monthly basis.
It is imperative that they buy into the idea in order to implement it.