This article reports on the development and psychometric evaluation of a new teacher observation instrument to systematically assess young primary school students’ well-being and detect potential indicators of psychological trauma, the RaPTOSS: Risk and Protective factors Trauma Observation School Situations. The RaPTOSS is developed specifically for preschool and early primary school teachers, because of the limited abilities their young students have to verbally reflect and report on their well-being and stressful experiences. We examined the factor structure, internal consistency, convergent validity and criterion validity of the RaPTOSS in a sample of 406 4- to 8-year-old regular education students. Exploratory factor analyses yielded a four-factor structure for the risk items with the following factors: Withdrawn, Dysphoria, Destructive and Inattentive. The protective factor items revealed a four-factor structure closely aligning the theoretical factor structure: Safety and Relations, Self-Image, Everyday Life and Self-Regulation. All factors exhibited good to excellent internal consistency. Correlations between the RaPTOSS and existing measures of psychosocial well-being without a specific focus on trauma-related behavior demonstrated good convergent validity. Criterion validity was supported, as indicated by moderate to large positive correlations between teacher worries about a student and RaPTOSS risk factors, and moderate to large negative correlations between teacher worries and RAPTOSS protective factors. We conclude that the RaPTOSS is a promising measure for trauma-informed teaching and research, although future research is needed to establish its psychometric qualities in specific samples such as children diagnosed with PTSD and refugee children.