Classroom-based physical activity (CBPA) can significantly benefit students' health and educational outcomes, but many teachers do not utilize CBPA. This study examined teachers' perceptions about the value and impact of several approaches to support CBPA implementation, and teachers' weekly self-reported CBPA use. Interviews were conducted with 35 classroom teachers (including those using and not using CBPA) at two public elementary schools, and CBPA tracking logs were collected on a weekly basis. Interview transcripts were interpreted through key domains within implementation science. On average, teachers reported using one activity every other day. Interview data revealed that utilizing professional collaboration time for peer-to-peer feedback and getting informal support from the school's physical education teacher both have some promise for increasing implementation of CBPA. However, teachers largely felt these strategies were unnecessary. Explicit administrator support was reported by teachers as the most promising mechanism for increasing their CBPA implementation.