"I Need..." Rather Than "You Need..."
As a rule, I-statements are more effective than you-statements when redirecting students. If, for example, you say, "You need to sit down," some students will take exception: "How would you know what I need to do? Don't tell me what I need to do." Other students will justify their action: "I was just sharpening my pencil." (Never mind that they were shooting jump shots at the trash can or socializing by the time you confronted them.) And then there are those kids who will say nothing at all, but instead try to show you (and their classmates) who's boss by shuffling or strutting or stomping back to their seats. In any case, it'll take all the self-restraint you've got to avert a drawn-out conversation if not an all-out power struggle.
By contrast, telling students firmly yet calmly, "I need you to return to your seat please," carries little if any potential for eliciting defensiveness or defiance. Instead, students are more likely to accept your request for what it is: an authority figure asking them to do what they themselves know is the right thing to do.
So if you're looking for cooperation from students rather than confrontation, tell them what you need rather than saying "you need..."
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