In this study, we examined the development of mathematics achievement in children attending extracurricular activities intensively in comparison with the development in a control group of children attending only the obligatory hours of school instruction. In addition, we investigated the question of possible effects of intensity of attendance and quality of extracurricular activities on achievement in students attending extracurricular activities from the end of Grade 1 to the end of Grade 3. This was a longitudinal study of a sample of N = 295 students in the first years of school at 35 primary schools in the German-speaking part of Switzerland. The results show that attendance in extracurricular activities had a positive effect on the development of students' achievement. Students who attended extracurricular activities intensively in Grade 1, students who attended high-quality extracurricular activities, and especially those who did both, had greater improvement.