In the United States, many classroom teachers also teach physical education (PE). However, there is a dearth of evidence- and standard-based PE programs designed to support classroom teachers to deliver PE effectively in schools. The purpose of this study was to establish proof-of-concept for the Pocket PE 3-5 digital app in school settings with 10 third- to fifth-grade classroom teachers. We assessed fidelity of program implementation, measured via observations of PE quality. Students used wrist-worn heart rate monitors during Pocket PE 3-5 lessons to measure time spent in moderate to vigorous physical activity. Program feasibility was primarily assessed through teacher-reported surveys of usability, satisfaction, and acceptability and exit interviews. Mean PE observation scores were 18.6 (SD = 1.5) on a scale of 5 to 20. On average, students spent 56.7% (SD = 13.1%) of class time engaging in moderate to vigorous physical activity. Mean survey scores, reported on a 5-point scale, were 4.5 (SD = 0.6) for acceptability, 4.8 (SD = 0.4) for usability, and 4.7 (SD = 0.7) for satisfaction. Teachers liked how easy the app was to use but mentioned some technological challenges. This program evaluation study established the proof-of-concept for the Pocket PE 3-5 elementary school PE program.