Self harm is no joke.
You always hear how self-harmers are "Emo" or "Attention Seekers", but self-harm isn't a joke. It takes a lot to get to the point of turning mental pain into physical pain. When you make fun of them, you're just making them pull down their sleeves and hide their skin even more. You're pushing them further and further into a hole, and if you don't be careful with what you say, that hole will turn into their grave.