Not all students have equal opportunities to learn from effective teachers, and students from historically disadvantaged backgrounds are disproportionately more likely to be taught by ineffective teachers year after year. However, the cumulative experience of being taught by (in)effective teachers has received less attention. Utilizing data from Tennessee's Student/Teacher Achievement Ratio and value-added modeling, we found that cumulative exposure to highly (in)effective teachers significantly impacts students' achievement. Students consistently taught by highly effective teachers for three years demonstrated approximately seven months more learning growth compared to those taught by ineffective teachers for the same period. The findings emphasize that the cumulative effects of prolonged exposure to highly effective and ineffective teachers can widen existing disparities in academic achievement. We conclude that teacher effectiveness, as well as access to and distribution of effective teachers, should be viewed from a cumulative perspective to better understand, effectively address, and reduce educational inequality.