Social isolation of school children is increased when teachers feel unable to respond to children's report of witnessing or experiencing aggression at school. Teachers can find it difficult to respond adequately to children who report aggression when the aggression is deliberately hidden from the teacher by the perpetrator. This study aimed to explore, and then confirm, the latent factor structure of the Student Experience of Teacher Support Scale (SETSS), a new instrument designed to measure children's experience when reporting aggression, including aggression that may have been cleverly hidden from school teachers. This paper reports two phases of the study (grades 4 to 6, Perth, Western Australia): sample 1 (N = 174) and sample 2 (N = 642). Qualitative methods informed instrument design. This paper describes the quantitative phases: first, exploratory factor analysis identified a two-factor model, student "experience" and "heard" by the teacher; second, confirmatory factor analysis and construct validity were conducted. The SETSS has a two-factor model of sound psychometric fit. The teacher's response to children who report aggression can be experienced by children as supportive; alternatively, children may experience increased harm. The harm experienced by victims of aggression is increased when aggression is deliberately and intentionally hidden from teachers by the perpetrator. This adds to the empirical understanding of factors that predict bullying, the identification of harm that results from bullying, and will help track the effectiveness of interventions and prevention and management of this significant health threat.