A bully’s joke needs an audience to work. Keeping a straight face takes away their stage.

A joke like that is a performance, and the laugh is the payment. Without the payment the whole thing stalls, which is why the person telling it keeps scanning the room. Holding your face still is not passive. You are quietly removing the part that makes it work, and that is something you can do without a confrontation or an audience of your own.

If you want to do more, turn and say something ordinary to the person being joked about. Then tell an adult you trust if it keeps happening. None of this is yours alone to fix.