This student-centered video study examines the generalizability and reliability of video observations of behavioral engagement of students, taking into account the active work phases (partner work, group work, plenary) of a third year primary school class (N = 20 students). We used an adapted version of the Munich Attention Inventory (Helmke & Renkl, 1992) to assess the behavioral engagement of all learners during the active work phases in one-second intervals by four independent raters. In educational psychology, video studies are often used to describe and analyze classroom processes – although they are considered as being very resource-intensive and costly. Therefore, in order to enable efficient and economical design planning of studies this study investigates up to how many seconds the behavioral engagement of students can be reliably observed. In line with our expectations, the results of the G-Study show that individual learners variance explanation was 18.85% across all work phases. The time in seconds (2.67%) explains only a marginal part of the variance. There remains a large proportion of unexplained residual variance (78.48%). The reliability coefficient with Φ = .99 is very good due to common standards. The decision study (D-Study) shows that results are reliable even when observing just 30 seconds (Φ = .87). The results indicate that student-centered video analyses offer a valuable way to capture the behavioral engagement of all students, in that information on all learners is available throughout the entire lesson. The results also contribute to an economic design of student-centered video studies on behavioral engagement.