In collaborative learning scenarios, the role of the instructor is vital. This aspect directly carries over to the concept of collaborative multiplayer Serious Games, where a group of players is learning together using a Serious Game. In this paper, the authors propose a novel approach for integration and support of instructors in collaborative multiplayer Serious Game scenarios. Their approach considers instructor tasks and responsibilities. It defines an interface for 3D action adventure-like games, defining relevant game information and adaptation access. It further includes a Game Mastering framework for orchestrating and adapting such games at runtime via an instructor. The concept was implemented on top of an existing Serious Game (Escape from Wilson Island) and evaluated in a user-centric study with N = 40 participants (age m = 23.39; SD = 3.05). Results show that an instructor using our framework in a collaborative learning scenario can positively influence players' game experience (p<0.05) and success towards the intended goals, as well as recognize and counteract problems at runtime.