Effective classroom management is critical to supporting student learning and success in school. However, little research has focused specifically on the impact of classroom management on student mathematics achievement, especially in the early elementary grades or in a small group (i.e., Tier 2) context. To address this need we investigated the relationship between classroom management and student mathematics achievement within the context of ROOTS, a validated Tier 2 kindergarten mathematics intervention taught by paraprofessionals (Clarke et al., 2012). Results show that the quality of classroom management within the ROOTS intervention predicted gains in student mathematics achievement from pretest to posttest. These findings have critical implications for practitioners, including the importance of integrating effective classroom management techniques into Tier 2 mathematics intervention groups and supporting paraprofessionals' knowledge and use of effective classroom management. [This paper will be published in "Psychology in the Schools."]