Grounding seems like an extreme response, and I’m a pretty big meanie. You seem to be lot more hurt than the kid is. Just tell him not to joke about that, and strike one privilege for the day if they do.
It’s really not that terrible!!!
The school administration are idiots. I’ve had completely unprintable things yelled out bus windows at me by high schoolers when I run outside that would make a construction worker blush. What he said was a lot better than that!
All teens are morons some of the time. Some are morons a lot of the time. Your job is to help them outgrow it!