Years ago, at the end of one of my daughter's school years when she was in middle school, I wrote a letter to her social studies teacher, whose class she loved. She didn't much like school in general, although she was a good student- she just thought most of it was boring and they were trying to push the kids into some kind of mold and she was quite an independent thinker.
I thanked him for his teaching style and that his class had been the highlight of school that year for her. When I next saw him, he told me he had never, in all his years of teaching (he wasn't young) gotten a letter like that from a parent, only complaints. My gratitude earned his.