I am having a hard time controlling students while they are in centers. This is problematic, as most of the time when kids are in centers, I need to devote myself to the one group that I am teaching. For example, when I hear general chatter, it is difficult for me to pinpoint and correct the offenders accurately. Any ideas on how to ensure that all students stay on task? And how to control the class while still devoting attention to my instruction group?