Hi Elly, I think we have all had experiences like that. It is a little more difficult when you are acting as a substitute and you do not know the students. In my ground classes, when that has happened, I start by requesting politely that they quiet down and if they want to engage in private conversations, to please do so outside of the classroom. If it does not abate, I basically tell them to leave the room until they can maintain the appropriate decorum of a college student. The college where I teach fulltime, offers a great deal of support to faculty to deal with disruptive students. They actually encourage you to write the student up for this type of conduct. In the years I have been teaching at the college level, I have written students up only a handful of times.
I hope this helps, Maureen O'Connor
