THEY WON'T STOP TALKING

Help! I'm a fourth year sixth grade teacher at a rural school. The students just will not stop talking. During instruction, during tests, quizzes. After I've asked them to stop. After they've lost time off recess. They just will not shut up. I'm exhausted and I've never experienced anything like this. What can I do? They know the expectations, I remind them constantly when it's acceptable to talk and when it isn't.

To be clear, it isn't all of the kids. It's about 5 out of 16 in each class.

Any ideas would be very much appreciated!