This study used a social presence construct derived from the Community of Inquiry (COI) framework, with cohesive, affective, and interactive components, to explore online discussion in an Advanced Placement course for rural high school students. The COI is based on the underlying theoretical framework of social constructivism, where interaction is a key component of learning. The study focuses on the contributions of students and online teachers to social presence in online discussion, and explores student dropout through the lens of social presence. Teacher social presence behaviors tended to be cohesive in nature, while students' social presence behaviors were primarily affective and interactive in nature. The study found that compared to their peers who completed the course, many students who dropped the course were more likely to use sarcasm, complain, or disagree with other students. Each teacher demonstrated unique patterns of social presence behaviors. There were also distinct patterns of student dropout characteristics associated with high social presence and low social presence teachers. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]