Group trial-based cognitive training (G-TBCT) is an adaptation of trial-based cognitive therapy (TBCT) which, in turn, is derived from standard cognitive-behavior therapy (CBT). CBT is recognized as an effective treatment for several psychiatric disorders. Core belief (CB) change seems to play an important role in this regard. However, little is known about methods and techniques specifically used to change dysfunctional CBs in children and adolescents. We present here an approach used as a preventative tool to help adolescents change their dysfunctional CBs in an engaging fashion, by asking them to role-play Detectives of the Mind to uncover such CBs; Attorneys of the Mind to help them evaluate if such CBs are true; Judges of the Mind to help them take distance and become metacognitively aware of the critical voices represented by internal prosecutors and sue them; and finally Masters of the Mind to help them make decisions that help them choose assertive and ethical behaviors. In this paper, we introduce the idea and principles of GTBCT, and, more specifically, the trial-based thought record, a method that was designed to help adolescents restructure dysfunctional CBs about themselves.