This paper uses a novel data on both students' and teachers' online participation with students' administrative academic records and characteristics. The first chapter provides evidence that pre-pandemic lower-achieving students benefitted of the extra time they got during the Spring 2020 lockdown by doing more homework and completing the work assignments that counted towards the final grade, improving their academic performance relative to their pre-pandemic performance (relative to the change observed among higher-achieving students). Using difference-in-differences models and event-study analyses, both with individual fixed effects, we find that lower-achieving students' Spring 2020 GPA improvement relative to their pre-pandemic academic performance (relative to the change in GPA observed among their higher-achieving peers) is driven by a relative increase in their time studying measured by student's online logging, signing into the platform, mouse clicks, and homework postings. The second chapter examined the effects of experiencing unexpected lockdowns more often on students' study performance and study effort. This chapter finds that students with more experience in lockdown while registering for classes can adjust to the shock better by performing better in course grades and investing more study time. Also, female students and younger students are the major forces driving such differences. The third chapter incorporates teacher value-added models (VAMs) and provides evidence for the effects of teaching assistants on students' academic performance. This chapter finds that although teaching assistants are not directly teaching students, they still have high impact on their academic performance through communication. Value-added estimates for teaching assistants differ by their efforts and experience. High quality teaching assistants can help students to achieve a higher course grade and reduce the probability of failing courses. Our results indicate that teacher value-added can be partially explained by their efforts and attitudes toward students' study, so policies should be implemented to incentivize teachers' efforts to improve students' academic performance. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]