The massification and diversification of the student population who enters university has become a challenge for Higher Education institutions. A better consideration of this diversity could help to gain a greater understanding of students' achievement process, and this requires observing situations where this diversity is substantial. Thus, in France University Institutes of Technology (IUT) constitute a promising context. This study aims at analyzing to what extent the diversity in student entrance backgrounds and student engagement could explain academic achievement. Analyzing the influence of these two types of variables on achievement allows to question the secondary education determinism on academic achievement. The impact of background diversity was estimated both at the individual level and at the contextual level (through class composition). To do so, multilevel analyses were performed among a sample of 748 first-year students (average age 18.43 with standard deviation of 0.98) from 50 classes in 11 IUT departments. Student engagement was measured using the "Approaches and Study Skills Inventory for Students" (ASSIST) and the "Strategies for Learning Questionnaire" (MSLQ). The results show that engagement in learning and academic background are both important individual factors to understand differences of students' achievement. Moreover, classroom composition explained a significant part of student achievement differences among the classes. Student achievement differences due to the academic background were important enough not to be fully compensated by student engagement. Implications of the findings for practices and further research on the first-year experience at University are discussed.