My co-op teacher says to wait, and look at the ones who are talking, until the entire class is quiet. I plan on talking about the noise at the beginning of the period on Monday. I am student teaching at a large school. There is only seven minutes between classes. The transition from class to class is critical.
Time is of the essence.
I tell my students that for everytime I have to say something to them about the noise, they'll have to remain in there seat for 10 seconds after the bell rings. Every warning adds 10 seconds to the time they have to remain in their seats after the bell rings. It works for me.