Flipgrid is a free video discussion tool that allows teachers to set up discussions for students and allows students to send video responses back.
Teachers create “grids” (i.e. a class) and topics within those grids.
Teachers can also assign text feedback to a student’s video.
See this video on how teachers can get started:
More information on using this tool can be found here: http://blog.flipgrid.com/gettingstarted