This article examines the effects of changing teacher self-efficacy on student achievement. Results from a quantitative study of mathematics teachers in a USA mid-Atlantic state showed that teachers experienced reduced self-efficacy when working with English language learner students (ELLs) relative to non-ELLs. Further, the article explores professional development opportunities for strengthening teachers' effectiveness with ELLs. The study utilized a four-part survey instrument based on the "Teacher Sense of Efficacy Scale" (TSES). All participants (n = 181) were mathematics educators from the highlighted state, and 94 % of them were preK-12 public school practicing classroom teachers. Study results indicate that teachers' participation in professional development on ELL instruction is positively correlated to their heightened sense of self-efficacy. This finding points to continuing education as a potentially viable approach to reducing academic challenges for ELL students in mainstream mathematics classrooms by developing ever more effective practicing teachers.