Adam Voigt from Real Schools in Australia delivers a wonderful webinar on how to run classroom circles. Basic tenets include:
1) They must serve your learning goals
2) Engagement is everything
3) Less Really Is More
4) Position, Position, Position
5) Make them scheduled and flexible; and
6) The Special Sauce. More than others, he emphasizes using circles for pedagogical reasons and keeping them short and sweet.
https://www.youtube.com/watch?feature=youtu.be&v=ZtRdxzPSweo&app=desktop