While they distanced themselves from Brian and Claire, Allison, Bender, and Andy remained friends. Thanks to some outside influence both Bender and Allison turned their lives around then, at some point, hook up. Throughout the rest of their term in High School they eventually made new friends. Although it could be possible that Billy's a member of Bender's crew that he'd hang with before the events of The Breakfast Club and still hang with him despite Bender's change of lifestyle. Which could explain how Bender's the only one of the group that can put up with Billy's actions as long as he has and still help him out. Because he has been there.
The movie is a chronicle of the three members of the Breakfast Club's life outside of high school but with a new cast of friends that replaced Brian and Claire.
