Voice note homework for a language school

Design voice note homework for an online language school. Students record a 30 to 120 second answer on their phone or laptop and submit it; their teacher plays it back, leaves a written or recorded comment, and gives a mark. The school has 20,000 students who submit about 15,000 recordings a day, mostly in the evening, often from phones on weak mobile connections. Browsers record in different audio formats. Recordings are deleted one year after the course ends. Describe the upload flow, storage and format handling, playback, and deletion.