How can I block facetime group calls from specific people without blocking every single person in the group?

How can I block facetime group calls from specific people without blocking every single person in the group?

My 8 year old son gave my phone number to a friend and now my phone number is included in multiple facetime group calls with different combinations of about 25 different classmates. My son's apple ID is also included in these groups. I've blocked every single one of their numbers except my son's (I can not block him because he needs to be able to contact me). I've read that the facetime calls will continue to come through as long as there is someone in the group that I haven't blocked. I can't disable facetime or delete facetime because I need it for my son to contact me. Also, I know that disabling facetime also disables imessage, which I prefer not to do.

I've repeatedly told my son to tell the other kids to remove me from their groups/create a new group without me. But they don't understand. It's very disruptive because they call repeatedly all day and night when they aren't in school. How can I prevent these group calls from coming through?