This study examined whether teacher-rated versus self-rated student temperaments are different in relation to the school grades in Maths and Mother language (ML) instruction in a nationally representative sample of Finnish Secondary School students (n?=?1,063, mean age 15.1 years). The results indicated that teacher-rated temperament was more strongly associated with students' Maths and ML grades than self-rated temperament. All the teacher-rated temperament traits were associated with school grades, accounting for 38% and 45% of the variance in Maths and ML, respectively. Of the self-rated temperaments, high task orientation and its components, high persistence and low distractibility, were associated with better school grades both in Maths and ML. Low reactivity and low negative emotionality were associated only with better ML grades, whereas high inhibition was associated only with better Maths grades. A temperament-conscious education needs to be taken into account in future teacher training.