We study an early effort amid the COVID-19 pandemic to develop new approaches to virtually serving students, supporting teachers, and promoting equity. This five-week, largely synchronous, summer program served 11,769 rising 4th-9th graders. "Mentor teachers" provided PD and videos of themselves teaching daily lessons to "partner teachers" across the country. We interviewed a representative sample of teachers and analyzed educator, parent, and student surveys. Stakeholders perceived that students made academic improvements, and the content was rigorous, relevant, and engaging. Teachers felt their teaching improved and appreciated receiving adaptable curricular materials. Participants wanted more relevant math content, more differentiated development, and less asynchronous movement content. Findings highlight promising strategies for promoting online engagement and exploiting virtual learning to strengthen teacher development.