In the United States during the later 1800s, unmarried young women from the east coast often went to the frontier areas of the United States to act as teachers in one room school houses. These girls were often from affluent families and did this in a pseudo-volunteer, altruistic fashion. Many did this for a couple of years before returning to the east coast to be married.

Teachers in these one room school houses were often paid very little or in some cases, entirely in food and board. Teachers would be given food and often take turns staying for a week or a few weeks at a time with different families in the community. There are stories of teachers even having to share a bed with their pupils!

So the idea of giving food to a teacher was sort of ingrained in many Americans' minds as normal. It wasn't really until men were encouraged to enter the profession that this idea of teachers being paid in food or doing as charity work started to erode.

The iconic nature of a student giving a teacher an apple was solidified by Bing Crosby when he sang a song called "An Apple for the teacher"(I think that's what it's called, correct me if I'm wrong.)

So through teachers being viewed as charity workers in need of food and Bing Crosby, the idea became entrenched in our folklore.

Answered in a previous thread