This study investigates whether daily Internet use time is related to the school performance of Korean adolescents. We therefore analyzed raw data from the 2009 Korea Youth Risk Behavior Web-based Survey-V (KYRBWS-V), in which 75,066 students from the middle-school first grade to the high-school third grade participated. We assessed the association between daily Internet use time and school performance by using multivariate logistic regression analysis adjusted for age, body mass index, education level of parents, and economic status of the family. For boys, the odds ratio (OR) [95% confidence interval (CI)] for recording ≥ average school performance and daily time spent on Internet use was 1.876 (1.752–2.008, p < 0.001) for ≤1 hour, 1.564 (1.467–1.667, p < 0.001) for >1–≤2 hours, 1.158 (1.079–1.242, p < 0.001) for >2–≤3 hours, and 0.759 (0.695–0.828, p < 0.001) for over 4 hours, when compared to those who never used the Internet. For girls, the OR (95% CI) for recording ≥ average school performance and daily Internet use time was 1.729 (1.607–1.860, p < 0.001) for ≤1 hour, 1.503 (1.399–1.616, p < 0.001) for >1–≤2 hours, 1.193 (1.100–1.293, p < 0.001) for >2–≤3 hours, and 0.809 (0.725–0.903, p < 0.001) for over 4 hours, when compared to those who never used the Internet. It was concluded that Korean adolescents who spend ≤3 hours per day on the Internet are predisposed to an increase in performance levels at school, whereas those who spend over 4 hours per day on the Internet are predisposed to a decrease in school performance levels.