National and international assessments consistently report gender differences in mathematics, with boys scoring higher than girls. This study aims to explore whether the gender gap has been overcome in mathematical competence; as well as to check what similarities and differences there are according to the school effectiveness, educational stage, and the Diagnostic Assessment (DA) editions in the Autonomous Community of the Basque Country (ACBC). The study follows a descriptive design with a quantitative methodology, analysing census data on the mathematical competence of 4th grade primary and 2nd grade secondary students during the most recent editions of the DA. The results confirm that mathematics achievement is determined by both the gender gap and school effectiveness. Even so, some positive trends can be observed in the latest editions of the DA and, above all, in the secondary education stage to close the gender gap. The study highlights the need to develop mechanisms for measuring equality in schools in a systematic manner, and reviews some of the strategies and action plans currently being implemented to foster gender equality in the classroom.