Observational measures can add objective data to both research and clinical evaluations of children’s behavior in the classroom. However, they pose challenges for training and attaining high levels of interrater reliability between observers. The Behavioral Observation of Students in Schools (BOSS) is a commonly used school-based observation instrument that is well adapted to measure symptoms of attention deficit/hyperactivity disorder (ADHD) in the classroom setting. Reliable use of the BOSS for clinical or research purposes requires training to reach reliable standards (kappa ≥ 0.80). The current study conducted training observations in one suburban and one urban elementary school in the Greater Boston area. To enhance interrater reliability and reduce training time, supplemental guidelines, including 30 additional rules to follow, were developed over two consecutive school years. The complete protocol was then used for training in the third school year. To reach sufficient interrater reliability (kappa ≥ 0.80) during training, 45 training observations were required in the first year while, in the third year, only 17 observations were required. High interrater reliability was sustained after training across all three school years, accumulating a total of 1,001 post-training observations. It is estimated that clinicians or researchers following this proposed protocol, who are naive to the BOSS, will require approximately 30 training observations to reach proficient reliability. We believe this protocol will make the BOSS more accessible for clinical and research usage, and the procedures used to obtain high interrater reliability using the BOSS are broadly applicable to a variety of observational measures.