An increase in the educational achievement of girls at the same time that the number of female teachers in primary school education is rising has led to the discussion whether boys are disadvantaged by the lack of male teachers. The Netherlands Twin Register identified a unique sample of 100 12-year-old monozygotic twin pairs discordant and 396 boy-girl twin pairs concordant for teachers' gender. School performance, as rated by the teacher, an educational achievement test score and teacher-rated ADHD behavior were similar for students with male or female teachers. In spite of the increase in the number of female teachers, boys still outperformed girls in arithmetic, while girls scored higher on language and reading. Boys demonstrated more ADHD behavior, but this was independent of teachers' gender. Therefore, increasing the number of male teachers in primary education may not be as effective to close a possible gender gap as suggested by some.