This paper describes the development of the "Flourishing Classroom System Observation Framework and Rubric," which provides a framework and practical approach to defining and describing multiple interconnected observable characteristics of a classroom system that individually and together can be targeted to cultivate collective flourishing within schools. Beginning with a working theoretical model based on existing literature, a three-round Delphi study was used to develop the framework and related rubric. In round 1, 35 experts answered open-ended questions regarding observable behaviours of collective wellbeing in the classroom. Analysis of responses resulted in a framework with five dimensions, each with three sub-dimensions. In round 2, 23 experts sorted and categorised statements that potentially described each of the 15 sub-dimensions. Analyses created definitions of each sub-dimension and statements describing how they manifest for teachers, students, and the class, along with a description of the learning environment for each dimension. In round 3, 18 experts reviewed the definitions and descriptions, resulting in a final 15-dimension framework with a related rubric of 45 descriptive statements. The resulting framework and rubric provide an organising structure to identify observable system elements that shape a flourishing classroom culture.