Flipgrid
Due to Ed Law 2-d, students should NOT create their own accounts. You, as the teacher, can create a teacher account and share a link with your students so they can post on a Flipgrid that you created.
Getting Started for Teachers
Getting Started for Students
How to use Flipgrid for Teachers
How to use Flipgrid for Students 2020
How to Make a Whiteboard Video in Flipgrid