How do I explain to my daughter that being mean is wrong

I never thought of my daughter as mean. I always thought she was very opinionated and some may say bossy, but I think that's good in a women. We should be teaching our young ladies to be opinionated and how to be a boss. She's 13 and a situation happened at her school. There was a special needs child at her school that poo'd himself and she was one of the kids that were making fun of him about it. The teacher st the Junior high said it wasn't light teasing, she and a few others were full on "harassing" him. In my personal opinion I think using the word harassing to describe a bunch of 13 14 year olds is a bit much. I'm not gonna ground her over it because she's only 13. I'm sure she's not used to seeing an 8th grader mess his pants. However I do want to teach her that being mean is wrong. How can I go about doing that?