Assessing the degree to which interventions are implemented in school settings is critical to making decisions about student outcomes. School psychologists may not be available to regularly conduct observations of intervention implementation, however, their data may be used alongside other methods for multi-informant assessment. Teacher self-report is a commonly used and feasible assessment method. Students have been trained to implement interventions with their peers in instances where traditional adult interventionists were unavailable. This exploratory study investigated the accuracy with which classroom teachers and middle and high school students assessed implementation of the Good Behavior Game and the impact of performance feedback on their accuracy. Results indicated that most students and teachers were able to provide accurate assessments of treatment integrity compared to researcher direct observation; however, some required performance feedback to do so. These findings suggest that multi-informant assessment may be a feasible and accurate way for school psychologists to collect formative treatment-integrity data in the classroom. Limitations and future directions are discussed.