Background: Studies have shown a relationship between students' health and their academic achievements, but whether health of classmates and schoolmates impacts individual students' school achievement is less known. We studied these effects on students in lower secondary school in Finland. Methods: Students (seventh grade, age 12-13 years, N = 7779, 123 schools, 565 classes) participated in a classroom survey measuring health (externalizing and internalizing problems, daily health complaints, and long-term illness) and academic achievement. Academic achievement when leaving school (15-16 years) was obtained from the Finnish national application register on upper secondary education. Three-level (student, class, and school) multilevel regression analyses were executed. Results: All health variables predicted academic achievement at leaving school at the student level and externalizing and internalizing problems at the class level; better health predicted better achievement. Students' health at the school level was not related to academic achievement. The effect of class-level health on academic achievement was as strong as the effect of student-level health. Conclusion: Both student and classmates' health at the beginning of lower secondary school contribute to academic achievement when leaving school.