This research aims to examine the effect of birth month on student achievement as well as social and spiritual development in the classroom. We designed this research as a mixed embedded experimental design, one of the mixed research designs. For the experimental group, first-grade students in a primary school in Aziziye District of Erzurum province in Turkey were classified according to their months of birth. For the control group, three classes formed from different schools and cities in a mixed manner were selected through purposive sampling method. Pretest and post-test applications were performed at different time intervals, and the data were analyzed using One-Way ANOVA and a repeated-measures t-test. In the qualitative part, we conducted interviews with the experimental group teachers. Based on the results of the research, there is evidence to suggest that the month of birth affects academic achievement. So much so that, the academic achievements of the 69-74-month-old group classified according to the month of birth were quite high, but the academic success of the 69-74-month-old students in the mixed classes is low. Mixed classes were academically homogeneous. They were also very healthy and in good condition socially and spiritually. These homogeneous groups, which were classified according to the month of birth, prevent students from being overwhelmed and lost, and facilitate the work of teachers. It can thus be suggested that first-graders in primary schools can be classified in terms of birth month and developmental levels in order to mitigate the effects of birth month.