There is no uniform definition of bullying under the law. (thebullyproject.com)
A clear definition of bullying with examples. (cyc-net.org)
Have them define what bullying means to them, then present them with the following definition of bullying: «Bullying is when a person hurts someone's body, feelings or things, or frightens someone on purpose again and again.» (myplaytherapypage.com)