Just give a couple of them detention and the problem will go away. I
am teaching a section of 6th and one of HS Art I this year, in addition
to K-5 (the 6 & HS come over to the elem). My 6th graders would not
stop talking, even when they were engaged and enjoying what they were
doing. So I gave a couple of them detention. Shocked everyone so much
that it has stopped! They know now that when I say they are getting too
loud that I mean it, and they DO get quiet. You may make a couple of
them "mad" at you, but your point will be received. Good luck!