Previous research points to digital attention training as a potential remedy for the growing levels of distraction and emotional distress that adolescents experience. However, no studies with a comparison group have been conducted in high school settings to assess the feasibility and efficacy of digital attention training. Using a two-group, pretest/posttest design, this study examined the effect of an online course called Finding Focus. Across three U.S. high schools, N = 197, classrooms were assigned to either continue with school- as-usual or to complete 2.5 hr of attention training over the course of 22 days. At pretest, data collection via online surveys showed that 77% of students reported focusing less often during class than they believed they ideally should. Compared to those in the control condition, students in the intervention condition reported at posttest adopting a stronger growth mindset regarding their ability to focus (Cohen's d = 0.42) and greater confidence that they knew how to train that ability (d = 0.89). At posttest, those in the intervention condition also self-reported less mind-wandering during class (d = 0.31) and daily life (d = 0.42), as well as higher classroom focus (d = 0.29). The intervention also led to improvements in perceived emotional regulation (d = 0.37). Collectively, these results suggest that digital interventions may be a promising and scalable avenue for improving adolescents' attention and emotional well-being. [This is the online version of an article published in "Technology, Mind, and Behavior."]