It’s a cliche that goes back a century or more. I don’t know if kids really ever did that; maybe in our predominantly rural communities, a little bit of fresh produce from the family farm was a standard courtesy for the teacher, who may not have been paid very much in cash. The cliche has to do with ingratiating oneself with the teacher. It became a graphic convention that tends to perpetuate itself, so that people who’ve only ever seen it done in a cartoon or a Norman Rockwell painting put it into their own illustrations, creating an entire stereotype of something that nobody’s ever seen (just like the little green Martians we’d all recognize if we ever saw one).