The identification of reliable and valid means for evaluating the effectiveness of school-based treatments and completing diagnostic evaluations of middle school aged students are needed. The present study examined the inter-rater agreement of teacher ratings and the relationship between ratings and observational data in a middle school setting. The data are interpreted in the context of differences between a secondary and elementary school setting. Teacher ratings and observational data were collected regularly over the course of two academic years for middle school students diagnosed with ADHD. The results indicate low rates of inter-rater agreement as well as low rates of agreement between teachers and observational data, and between observational data collected in different classrooms. Inter-rater agreement was lowest in late fall and gradually increased over the second half of the year. Implications for conducting treatment outcome evaluations of school-based treatment programs and diagnostic evaluations are discussed.