Excellent post geekesse, think it really sums it up. My eldest daughter became frankie when she was in her teens. This was back in the 1980's and it was just accepted as one of the annoying aspects of teens. Within a couple of years that had been dropped and I was more worried about that she was using good contraception as she moved between boyfriends.
Betty, sound as if your son and DiL are very good supportive parents, follow their lead. This has nothing to do with a particular pupil arriving at the school and, most definitely is nothing being taught.
It is just part of the modern times and teens trying to sort themselves out. Please try to use the name(s) they have chosen. And as for pronouns, a lot of teens these days prefer to be referred to as they/them rather than he or she.
One of my g. children is now on their THIRD name, another on their second. Not that hard to remember what name they like, in the past many youngsters shortened their name or used their second names, not really a difficult part of loving them to use the name they prefer to be addressed by.
Do ask them if they like to be addressed as her/him or they? They will appreciate this and you will rise in their estimation.
However, at 14 years old, they are starting to detach themselves from the people who were so important during childhood, and that very much includes grand parents, however close they may have been in the past.
You should be happy that your grandchild is developing and sorting out their own personality. No idea where it will take them, wherever it is - g.parents role is just to be loving and supportive in every way.