During academic activities, adolescents must manage both the internal distraction of mind-wandering and the external distraction of digital media. Attention training has emerged as a promising strategy for minimizing these distractions, but scalable interventions that can deliver effective attention training in high schools are still needed. The present investigation used a one-group pre-post design to examine the feasibility and outcomes of a digital attention training course at a public high school. The intervention was delivered with reasonably strong fidelity of implementation, with students completing 92% of the lessons and 79% of the daily exercises. At baseline, students reported mind-wandering more frequently during class than they multitasked, and mind-wandering was more negatively correlated with classroom focus. From pre-test to post-test (n = 229), students reported improved emotional regulation and reduced mind-wandering during daily life. Among the 76% of students who felt they paid attention in class less than they should, classroom focus improved significantly. During class, these students reported significantly less mind-wandering but slightly greater digital multitasking. During homework, they reported significantly less digital multitasking but only marginally reduced mind-wandering. Collectively, these results suggest that online interventions could be a scalable way of providing attention training in high schools, but that future work must consider the role of both mind-wandering and digital multitasking.