Flipgrid.

Flipgrid is a simple, free, and accessible video discussion experience used by millions of PreK to PhD learners, educators and families around the world. Flipgrid amplifies student voice and empowers learners of all ages to engage and share using the power of video.
Educators create and share discussion Topics with their learning communities. Learners simply download the app, enter their unique Join Code, and record short videos to share.