This is a study of the relationships between the student-body composition of the school, some characteristics of the scholastic process—culture and school climate—according to learners’ perceptions, and the mathematics achievement of students in the last year of high school in Argentina. The data used came from the 1998 National Census of High School Completion, carried out by the nation’s Ministry of Culture and Education. The file contains data for 135,000 students of 2,708 schools in 25 states. Multilevel linear modeling with three levels (student, school and state) was applied. A strong relationship was detected between mathematics achievement and the variables student-body composition and school process. When both variables acted together, the effect of other variables experienced a pronounced descent. Although reduced, the variables of the process influenced the student’s achievement. There was identified for future works a reference model which would evaluate other institutional learning factors.