One person speaks at the time. Don't laugh at others if it hurts them. Keep to the subject. It is very important to keep these rules during the first meetings. If, or perhaps more likely, when a violation occurs, stop the meeting and deal with the matter at once.
Some teachers use an object which is passed around in the circle.
The person holding the object is the only one allowed to speak. If someone laughs at another and it hurts them, deal with it in order to prevent it from happening again. Starting up the Class Council Finding your place in the circle. If the answer is NO, this is the time to change. Ask those who are not satisfied, where they would like to sit.