Background: Loneliness during adolescence has adverse consequences for mental health, education and employment outcomes. Yet, we know little about common correlates of loneliness among adolescents, making intervention work difficult. Aims: In this study, we (1) explore individual-, school- and country-level correlates of loneliness to help identify potential intervention targets, and (2) examine the influence of loneliness on academic performance. Sample: A total of 518,210 students aged 15 years from 75 countries provided self-reported loneliness data. Results: Using multilevel modelling, we found individual-, school- and country-level correlates of self-reported school-based loneliness, and showed that loneliness negatively influenced academic performance. Conclusions: Based on the findings, interventions that focus on enhancing social and emotional skills, increasing trust between teachers and students and changing school climate to be more inclusive are likely to be the most effective for adolescents; they should also be culturally sensitive.