Ninth-grade students (N = 381) completed a newly designed measure which assessed loneliness in four school contexts: classes, where they spend time between classes, lunchtime, and physical education. Students also indicated where they felt the most lonely at school and why. Assessment of loneliness in each school context was reliable. Loneliness varied across contexts, with girls reporting more loneliness than boys only in their classes. Loneliness was negatively associated with student's feelings that they belonged at the school and had positive relationships with their teachers. Open-ended data suggest that students' peer interactions influence their feelings of loneliness at school.