Talkative students

I got a new group of middle school students starting on Wednesday and they just won't stop talking. Things that I have tried so far:
-waiting patiently and not talking over them
-making it clear that I dismiss and not the bell (this allows me to quickly dismiss the students who weren't talking first)
-hold a few after both days
-asked individual students what consequences should follow if they continue to talk (thus they are creating their own consequences - this worked with the few I had trouble with in my last group)
-talked with my mentor (who sees these kids 4 times a day) who says she will bring it up at the student-led conferences they are having tonight and Monday

We will see how things go tomorrow. Several of them are kids who do not have the support at home and calling home would do nothing. So if things aren't better tomorrow, I'm not sure where I will go from here. There are some great kids in there I can tell but I can't see or hear them when the rest of the class is a three ring circus.