During puberty, young people shift their time-of-day preferences from morningness to eveningness. One of the main problems seems to be early school-start times, which force adolescents to start working at a given time that may be too early for them; and this, in turn, negatively affects school functioning. Here, we ask whether morningness-eveningness influences school performance as measured in grades. A total of 811 school students aged 10-17 years responded to our pupil morningness-eveningness questionnaire (PMEQ) derived from the German translation of the original MEQ. PMEQ scores decreased with age and indicated a shift towards eveningness around the age of 12 years. Gender differences did not exist. School achievement decreased with age and differed between boys and girls. After accounting for age, we found a highly significant negative correlation between average grading in the major subjects and PMEQ score, indicating that pupils with morning preferences performed better in school achievement. (Contains 2 figures, 1 table, 1 note and lists 3 online resources.)