Students often want allies in their arguments. Particularly at KS3 pupils do not have the emotional maturity to deal with insults or negative behaviours and they turn to you as an 'insitu parent'. Throughout my day I often hear comments like:
"He hit me!"
"She stole my ruler"
"Miiiissss....he sucked his tie and then wrang the spit out on my book"
This then escalates into a great big argument that you are expected to ajudicate.
One of the most effective comebacks I learned was a caring, but sharp, "What would you like to say to him/her about this?" This works because it shows the student that you are listening and willing to help solve the problem, however you are not taking over. Instead, you are showing the learner that they have a choice in how they deal with the situation and they can use words to defuse the situation.
Sometimes students don't know what to say. Here, I prompt them: "Could you explain how you feel?" or "What would you like her to do to repair this?" Occassionally the imaturity of a student means this is not productive but most of the time the two students work out their problem and you are free to get on with the business of focusing their learning back towards the topic at hand. Besides, there's nothing better than the next time this happens hearing a student turn to their partner and in the same caring, but sharp, voice say: "Anthony, when you wring that tie it makes me feel ill, please stop!"