Over the course of my years, I have learned the reasons behind the commandment. “Do not testify as a false witness”. Witnesses are the cornerstone of our judicial system, be it on the street, in court or even a child in a schoolyard. In laymans terms testifying as a false witness is called lying, however some lies are worse than others. When someone comes forward with a statement or an affidavit that they saw or heard someone do or say something they are believed. This gives them the power make or destroy a day, a week, or a life in some cases.
Whether it be something as simple as saying another student hurt you to the teacher or principle or going to the police and making up a story that someone did something they never did. The commandment “Do not testify as a false witness” applies to adults, children, in court and on the street. I have seen clients fired, persecuted, convicted, arrested, and imprisoned simply on the testimony of one false witness. It is equally important to teach children this lesson too. Children can be cruel, and allowing your children to be bullies and lies to school autherities about other children is equally as bad as testifying yourself as a false witness.
https://www.chabad.org/library/article_cdo/aid/900721/jewish/Negative-Commandment-285.htm