This article provides an empirically grounded analysis for two fundamentally different models of mathematics teachers' beliefs about student diversity in Russian secondary schools: exclusive and inclusive models. Although teachers' beliefs are considered a central factor for the differentiated approach, teachers' beliefs could be stereotyped and, consequently, the evaluation of a student's ability would be systematically shifted and decisions about the possibility of teaching a student would be incorrect. Semi-structured interviews with 30 mathematics teachers allowed us to investigate what criteria teachers claim to employ while classifying students in the classroom and what expectations they have for each group of students. It was found that within the exclusive model, teachers have an image of a "normal" student and use discrete categories for labelling students with reference to the "normality". Within the inclusive model, teachers tend not to match students with discrete categories; rather they prefer to compare a student only with herself or himself. Research findings are discussed in the context of a possible "fixed effect" on a student's development. However, there is a need for further investigation of a connection between teachers' belief systems, teaching practices and student achievement.