We present a conceptualization of student engagement based on the culmination of concentration, interest, and enjoyment (i.e., flow). Using a longitudinal sample of 526 high school students across the U.S., we investigated how adolescents spent their time in high school and the conditions under which they reported being engaged. Participants experienced increased engagement when the perceived challenge of the task and their own skills were high and in balance, the instruction was relevant, and the learning environment was under their control. Participants were also more engaged in individual and group work versus listening to lectures, watching videos, or taking exams. Suggestions to increase engagement, such as focusing on learning activities that support students’ autonomy and provide an appropriate level of challenge for students’ skills, conclude the article.