Smartphone ownership and use by adolescents have increased rapidly over the past decade and a half. Although some educators anticipated benefits of smartphones for learning, increasing smartphone use has raised numerous concerns in school environments, including the possibility that the mere presence of a smartphone reduces students' cognitive capacity for learning as a result of smartphone-related cognitive dual processing. This dissertation study sought to replicate mere presence effects of smartphone placement--a finding previously seen in college students--with a sample of high school students tested in a classroom setting. Eighty-five student participants recruited from a large, suburban high school completed four performance tasks in one of three randomly assigned smartphone location conditions--out of the room, in a bag, or on the student's desk. Results partially supported mere presence effects for more cognitively challenging tasks, but suggested that smartphone location familiarity, rather than proximity, may explain student performance differences. The study also explored the role of frequency of smartphone use, media multitasking, smartphone attachment, and smartphone dependence for academic and cognitive task performance. Implications for classroom learning, school policy, and post-pandemic digital wellness are discussed. [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.]