The learning context, consisting of the school children's families, teachers, and peers, has effect on their mathematics learning. The concern of students' socioeconomic status (SES) affecting negatively their learning outcomes is increasing worldwide. This study investigates whether Finnish elementary school students' SES affects their mathematical competence and success expectancy on individual and class levels. Additionally, the role of teachers' beliefs on mathematics learning and class composition on the mathematics competence and success expectancy on the class level is explored. To analyze the nested data from student questionnaires and mathematics tests, and their teachers' questionnaires, we used multilevel structural equation modelling with two levels (1, individual; 2, class). The results indicate that on the individual level, the gender and SES affect students' mathematical competence and success expectancy in mathematics. On the class level, the teacher's evaluations of academic class composition predicted students' mathematical competence, and the teachers' constructivist beliefs of mathematics learning and class composition regarding students' special needs predicted students' success expectancy. We conclude that students with disadvantaged SES need support on success expectancy to flourish in mathematics. On the class level, this support can be conveyed through teachers' constructivist pedagogical beliefs.