I will describe findings from a long-term study of online course-taking in secondary education, which is widely used in U.S. schools to help students make progress toward graduation. The study looked inside educational settings to observe how online courses are used and to assess whether students gain academically through their use, drawing on millions of records of online instructional sessions linked to student records. These records were also linked to data on student educational and labor market outcomes, including credits earned in high school, high school graduation, college enrollment, and labor market earnings. The students who were falling further behind educationally were more likely to be assigned to take their courses online, and they were also typically less prepared academically and fared more poorly in their education and post-high school outcomes. Limited resources both motivated the use of online course-taking systems in schools and constrained the implementation of recommended practices and instructional supports to support student learning.