<p>In Finland, research highlights a persistent academic achievement gap between majority population non-immigrant and immigrant-origin students. The present study examines whether and how social factors, such as the quality of teacher-student relationships, students’ sense of school belonging, and the schools’ proportion of immigrant-origin students, are associated with academic achievement among non-immigrant majority population, and first- and second-generation immigrant students in Finland. Using multilevel data from a Finnish sample (<i>N</i> = 10,239), this study investigates the associations between these social factors and reading comprehension test performance—a key indicator of overall cognitive development and cultural adaptation. Results suggest that the perceived quality of teacher-student relationship is positively associated with higher reading test performance for all students. Conversely, a strong sense of school belonging was positively associated with higher reading test performance for first-generation immigrant students. A higher proportion of immigrant-origin students at the school was negatively associated with reading test achievement only for first-generation immigrant students. Our study highlights the importance of fostering positive student-teacher relationships and a sense of belonging at school for narrowing the achievement gap and supporting positive educational trajectories for all.</p>