Keeping others on track.....
I think that grade level plays a big part in the type of intervention you design.. I know that intermittent rewards really work. If the individual or group does not know when the reward is coming, (interval of unknown length) then the behavior is strengthened. For example, in a negative sense, if you occasionally "give in" after your child begs or pesters, then the behavior of begging or pestering will be strengthened. In a postive sense, when a child is rewarded for an appropriate behavior, but doesn't know how frequently they will have to perform that behavior, then the behavior is strengthened as well, and they tend to increase the frequency of the appropriate behavior.
Initially, you might have to reward small approximations of what you want, say, most of the students, after a small incident, continue with their classwork. A whole class reward of homework pass, or extra time for something they like, in this case will identify the "good behavior". Next time, after an incident, if there is marginally improved responses, reward with something else, candy, whatever. Keep increasing the requirments, and don't reward every time, so they will not know when the reward is coming. This will continue to strenthen the behavior. Hope this helps.