The apple's use as a symbol for teachers began in frontier times. Teachers on the frontier frequently received apples from their students as a show of appreciation.
The families of children who attended frontier schools were responsible for feeding and housing the frontier teachers. Giving the teacher an apple contributed to her sustenance and demonstrated appreciation for teaching as many as 50 students at a time.
Apples remain a popular way to show appreciation to a teacher, even though the practice of feeding them has long fallen by the wayside. The phrase "apple-polisher" is synonymous with brown-nosing or attempting to curry favor with a teacher.