Here is a simple template you can use to write a complaint letter against fighting.
To Whom It May Concern:
When it comes to people hurting each other, it does nothing for both of them. The one throwing the punch may look like the better man but he ends up hurting someone. Meanwhile, the one getting hurt would bleed and suffer multiple injuries.
It is definitely time to stop any form of violence. Why can’t everyone just get along? If you get mad at someone, find another way to cool down. Don’t do something you are going to regret in the future.