Instruction plays a critical role in student success. However, most studies of teachers' effects on student learning focus on face-to-face settings. Some aspects of online courses, such as the choice of synchronous or asynchronous instruction and the course structure, might reduce teachers' influence on their students. This study of an online high school biology course offered by Florida Virtual School examined the variation in students' course segment (similar to semester) completion rates, end-of-segment exam scores, and time to segment completion that is attributable to the influence of teachers. Students' scores on the end-of-segment exam varied only slightly across teachers, while completion rates and time to completion varied more. [For the appendixes, see ED609865; for the study snapshot; see ED609866.]