Leah Binkovitz traces the tradition:
Held up as the paragon of moral fastidiousness, teachers, particularly on the frontier, frequently received sustenance from their pupils. "Families whose children attended schools were often responsible for housing and feeding frontier teachers," according to a PBS special, titled "Frontier House, Frontier Life." An apple could show appreciation for a teacher sometimes in charge of more than 50 students. … By the time American scholar Jan Harold Brunvand published his book, The Study of American Folklore, in 1968, the phrase "apple-polisher" was more or less shorthand for brown-nosing suck-up.
The Dish 