So, it's morning. We're in a portable. All the kids have finished getting snack. Some students finished snack and didn't push in their chairs. Other students simply didn't want snack, so they left without pushing in their chairs. I picked out one student who moved from one chair to another without pushing in her first chair.
Me: "Go back and push your chair."
Her: "But, everyone else didn't push in their chairs."
Me: "It's true that a lot of students didn't push in their chairs. But, everyone is supposed to push in their chairs. Push in your chair."
Her: (She pushes in her chair)
Me: "Thank you"
I wait a little bit and think about what she said.
Me: I have a suggestion for you. In the future, if a student doesn't push in their chair, tell them that they're supposed to push it in. That's the best you can do.
Her: And, if they don't, you could give them detention!
Me: Uhhh... maybe. I'll think about that.
Anyway, the bottom line is that, if a student tells you that everyone else didn't do a particular task, say that it doesn't matter because the task still needs to be done (i.e. in this case, pushing in one's chair).