A comprehensive, integrated, three-tiered model (CI3T) of prevention is a framework for proactively meeting students' academic, behavioral, and social skills. At the tertiary (Tier 3) level of prevention, functional-assessment based interventions (FABIs) may be used to identify, develop, and implement supports based on the function, or purpose, of target behaviors in their specified settings. FABIs have been shown to be more effective than interventions that simply increase reinforcement for desired behaviors and punish undesired behaviors. In this study, a FABI was conducted as a Tier 3 support in an inclusive high school implementing a CI3T model of prevention. Andrew was identified by his assistant principal as needing Tier 3 supports based on his low academic GPA, high number of office discipline referrals (ODRs), and ratings on the school's behavior screening data. A unique feature of this study was that it was conducted in Andrew's English class, which included a coteaching approach to instructional delivery. Three main questions were explored: (1) To what extent was the FABI effective in improving on-task behavior of a high school student taught in a cotaught, inclusive classroom?; (2) Was the intervention implemented as planned and viewed as acceptable by the participants?; and (3) Was it feasible for the special education teacher to assist with collecting data on student behavior in a reliable and feasible manner? Overall, the results demonstrated a functional relation between on-task behavior of a high school student with Attention Deficit Hyperactivity Disorder (ADHD) and the intervention put in place in an inclusive English class. On-task behavior started out at low levels during baseline and increased during the first intervention phase. When the intervention was removed, Andrew's on-task behavior immediately decreased to levels similar to those of the first baseline. Finally, once the intervention was reintroduced Andrew's on-task behavior again immediately increased and remained at high rates through maintenance. This study also supports the potential feasibility of teachers as data collectors. The use of momentary time sampling by Mr. Vega, the special education teacher, was shown to be dually beneficial. First, this study demonstrates that data collection using momentary time sampling can unobtrusively be done within a teacher's instructional routine, particularly in a coteaching model. Second, it provided the teacher with a simple and practical method of collecting data and monitoring student performance that can be implemented without the assistance of outside personnel. (Contains 4 tables and 1 figure.)