The purpose of this study was to examine principals’ and teachers’ perceptions of flexible ability grouping when used for students with exceptional mathematical promise in third and fourth-grade mathematics classes. Flexible grouping allows educators to match students’ readiness levels with instruction and to apply proper content and pacing. This study extended previous research on grouping practices and investigated flexible grouping in relation to mathematics instruction, both within-class ability grouping and between- or cross-class ability grouping. The type of research design was an explanatory sequential mixed methods approach, combining quantitative survey and qualitative interview data. The survey used a Likert-type rating scale and contained 23 questions. Participants reflected a purposeful sampling method, and included 86 elementary educators, consisting of 77 teachers and nine principals from five suburban school districts in Pennsylvania. After survey completion, a small group of 10 participants engaged in one-on-one interviews. Interviewees included two principals and eight teachers. Principals and teachers provided information about perceived effects of flexible grouping for students with exceptional mathematical promise, advantages and disadvantages of this practice, and planning and implementation aspects needed for successful delivery. Principals’ and teachers’ perceptions revealed significant results, including the beneficial effects of flexible grouping on academic performance and social and emotional behaviors for students with exceptional mathematical promise. Survey and interview data indicated that the advantages of flexible grouping as an instructional practice for students with exceptional mathematical promise outweighed the disadvantages. Some of the main advantages that surfaced in this study included high levels of student engagement, opportunities for higher level thinking, faster pacing of curriculum, and student preferences for working with peers of similar readiness and ability levels. Finally, this study explored aspects of planning and implementation necessary for flexible grouping at the elementary level. Results of qualitative data collection also revealed a need for meaningful professional development and training as well as sufficient teacher planning time through professional learning communities or grade-level meetings. Flexible grouping in mathematics should be considered a valuable and effective tool in meeting the needs of students, especially students with exceptional mathematical promise.