This would make a great procedure (Harry Wong). Count to five or hold up one finger at a time when the class needs to refocus. Discuss what occurs for each number at the beginning of the school year and eventually the reaction will be automatic.
There will always be a conflict in your classroom about how something is not fair and this poster would be great to have up in the classroom. When students start a confrotnation on conflict point at the poster and have each student read it aloud.