This research examines online course-taking in high schools, which is increasingly used by students falling behind in progress toward graduation. The study looks inside educational settings to observe how online courses are used and assess whether students gain academically through their use. Drawing on 7 million records of online instructional sessions linked to student records, we find mostly negative associations between online course-taking and math and reading scores, with some gains in credits earned and grade point averages by upperclassmen. Those least prepared academically and with weaker course-taking behaviors fared more poorly and were likely set back by online course-taking. Limited resources constrained the implementation of district-recommended practices and instructional supports, such as live teacher interactions and individualized content assistance.