I think it can be reasonably believed that the teacher must've meant it. She doesn't know the grandmother, but she does know the kids, and kids tend to be fairl

I think it can be reasonably believed that the teacher must've meant it. She doesn't know the grandmother, but she does know the kids, and kids tend to be fairly open about excitedly telling their friends that their grandma is visiting. Eg my niece's teachers know she really enjoys it when I visit despite never having met me because she draws me often. It can also just be seen as an appreciation towards the grandmother for being there enough for the kids to come to the meeting.