Background: Research suggests that physical activity breaks (ABs) during class increase students' physical activity levels and provide an academic benefit. This study evaluates a 3-year intervention aimed at encouraging teacher AB use. Methods: Thirty central Texas middle schools were assigned to 1 of 3 conditions: training-only ("Basic"), training plus facilitator support ("Basic Plus"), and training/facilitator support and a social marketing campaign ("Basic Plus SM"). Teachers completed surveys at end of years 2 (N?=?1039) and 3 (N?=?831) to assess exposure to program, self-efficacy, and frequency of AB use. Results: At end of year 3, teachers in facilitator-supported conditions reported increased exposure, self-efficacy, and use compared to Basic condition. Only 43.2% of teachers in the "Basic" condition reported receiving training in ABs compared to 84.2% and 90.6% in the "Basic Plus" and "Basic Plus SM" conditions, respectively. Additionally, a greater percentage of teachers in the facilitator-support conditions reported conducting ABs weekly ("Basic"?=?23.3%, "Basic Plus"?=?34.4%, "Basic Plus SM"?=?38.7%, at year 3; p?<?0.001). Conclusions: Despite perceived barriers, including fear that ABs will detract from instructional time, the intervention was successful in having a core group of teachers implement them weekly. More research is needed to increase the percentage of teachers implementing ABs regularly.