Purpose: We examined oral health behaviour and its association with school achievement among Finnish adolescents. Materials and Methods: This study is part of the Finnish national School Health Promotion study (SHP). The study population comprised a representative sample of Finnish 15-year-olds (N = 45,877). A questionnaire inquired about the respondents' school achievements and health habits (toothbrushing, smoking), background factors (age, gender, school type, family structure), and their parents' background factors (education, smoking). Chi-square tests and logistic regression models were used in the statistical analyses. Results: Better school achievements were associated with better oral health behaviour: 73.1% of students with the highest mean grades (9–10) brushed their teeth twice daily, compared to 33.8% of those with the lowest mean grade (6.9 or less). The lowest mean grade was associated with brushing less than twice daily, especially among boys (odds ratios (OR) = 4.1; 95% CI 3.6–4.7) when compared to those with the highest mean grade, but also among girls (OR = 2.3; 95% CI 2.1–2.7). Smoking among boys was associated with poor oral hygiene (OR = 1.3; 95% CI 1.2–1.4). Conclusion: School success is strongly associated with oral health behaviour among adolescents. Preventive treatment should be targeted especially at boys with poor school achievement and smoking behaviour.