The present research describes the development and pilot testing of a new instrument, the Responses to Interpersonal and Physically Provoking Situations Schedule (RIPPS), designed to measure the reactivity of students with and without attention deficit/hyperactivity disorder (AD/HD) in the naturalistic setting of the classroom. For this study, 29 pre-service teachers from one university graduate school of education conducted structured observations on two students each, one clinically diagnosed student with AD/HD and the other with no diagnosed disorder, resulting in 58 Year 8-11 students (aged 13-17 years) participating in the study. Each student pair was observed for 40 minutes, with alternating observational blocks of two minutes per student. Observational data in the form of responses to emotionally provoking events and the triggers to the responses were clustered together and systematically coded, resulting in four distinct categories for responses and four distinct categories for triggers. Students with AD/HD exhibited significantly more solitary off-task behaviours, interactional off-task behaviours, and challenging behaviours than their non-AD/HD peers. There were no differences between the students in the perceived severity of responses. For triggers, failure to begin assigned tasks and peer-initiated triggers were the most common, with nearly half of the solitary off-task behaviours being attributed to environmental distractions and over a quarter attributed to teacher behaviours. While the RIPPS is a relatively new instrument, important data have been gathered in ecologically valid contexts and provide the framework for further development of an instrument of this nature. (Contains 4 tables.)