The programming in the Stack is very different than in the younger groups. There are very few officially scheduled activities. Yes, there is a different schedule for the 13-15 and for the 16-17. However, in reality, the kids are all in there together (or at least they were for the 2 cruises that DD was in the Stack). They can listen to music, dance, do crafts, play PS2, whatever. THere is a long list of "you can do this anytime, just ask counsellor for supplies" activities....and very few scheduled activities. Even when there is a scheduled activity, each teen can decide to participate or not...and many of those activities are together.
So....technically the kids will be in different "groups," but in reality, they are mostly together. One caution--our cruises were both during the school year so there were relatively few teens. There may be more separation on cruises with more teens....but you still have the "hang out" feature where they are together.