It’s important to teach them the possible consequences of these types of “jokes” as the reason why you don’t tell them. You don’t want to parent through fear, but you do want to outline the facts of what can happen if this behavior continues. Today, one of those facts is juvenile detention when you’re making threats of harm to others on school property. I personally would also talk about how horrible the reality of what they are joking about is, and that joking about it makes the experience of victims seem unimportant (which it’s not).
Here’s help with impulsivity of speech and blurting:
ADDitude Community Moderator, Author & Mentor on Parenting ADHD, Mom to teen w/ ADHD, LDs, and autism