My 9th-grade students do it all the time. The main rule in my classroom is to not talk during any type of presentation. Usually, it means that students should shut their pieholes while I am delivering instruction. I don't spend more than six or seven minutes at a time talking, but I have rarely made it without multiple interruptions.
Let me know if you have any ideas, because I hate scolding the group for the sake of a few. And it seems to be counterproductive anyway; the few miscreants seek safety in the group. I suppose I continue to punish everybody in hopes of building a group culture, one that does not tolerate substandard behavior.
That's certainly the idea behind collective punishments in the Army. Teamwork, we are told, is the goal.
It must take more than nine months to develop.