Ok so maybe the teacher assigns the thank you note. But kids are honest and they tell you what they like and don't like.
I recently visited Chinle Middle School on the Navajo Reservation.
You know you have made an impression when a student takes out pencil and paper and begins to draw, inspired by your presentation, before you even leave the classroom.
Even when the thank you is a group effort you know the book was appreciated and the effort of a school visit worth every minute.