While there is no gap in math performance at the beginning of Grade 1, a gap in favor of boys appears and widens during the first year of primary school. Using standardized national assessments administered during Grade 1 (CP) to more than 2.5 million pupils in France between 2018 and 2022, we show that this relative drop in girls’ performance is observed for all the cohorts and most of the exercises assessed. The greatest drop-off occurs among the best-performing girls at the start of Grade 1 (those in the top 1 % initially). These girls lose an average of nearly 7 percentile ranks at the start of second grade compared with boys in the same initial percentile. The emergence of a gender gap in math performance during Grade 1 is observed across all social categories and family compositions, and throughout the country. Girls lose slightly less ground compared to boys in classes where the top student in math is a girl, and in priority networks public schools (REP or REP+). However, characteristics of the school environment explain only a small part of the overall dynamics, suggesting that girls are losing ground compared to boys in every strata of society.