The paper deals with the investigation of gender differences in performances in mathematics for Italian students at the end of lower secondary school. The study is based on a new large-scale assessment test developed and administered by the National Evaluation Institute for the School System. Given the evidence in the literature which favors males, performances of female and male students are compared using different approaches. Scores proposed by educational experts based on item subgroups were considered, while a model-based approach was used within item response theory. The results revealed a significant advantage to males in overall performance, while no meaningful differences were observed with respect to item domain and type. An interpretable item map was developed crossing expert opinions with IRT abilities, and plausible proficiency levels were defined. According to the map-based student classification, a relatively lower percentage of females fell into the highest proficiency groups with respect to males. (Contains 2 figures and 7 tables.)