This paper examines the presence of systematic differences in teachers' grading behaviour across gender and whether these can be attributed to teacher bias. This study measures these differences by comparing teachers' grades with national exams, which are externally and anonymously marked. Consistent with the literature, the gender gap in teacher grading is against boys. Using a dataset with gender gaps at class-subject level -- which allows to follow teachers in different classes over time -- this study shows that teachers' grading behaviour is not persistent across classes. Results suggest that gender grading gaps are explained by differences in students' behaviour.