I just finished up student teaching with a K class....let me tell you, I have never seen so much food being brought into the class before in my life, most of it was junk.

Let's see:
Patterns were made with marshmallows, skittles, and m&ms
Patterns with different colored/shaped candy corns
one week before halloween the kids got popcorn balls (not the worst thing but)
Once every week there was a star of the week, this child was to bring in his favorite snack for all his classmates on Wed. And it was usually cookies, cupcakes, etc.
Class holiday parties
They had a rewards system and when it got to a certain point they got an ice cream party.
St. Patrick's day they counted how many different marshmallow shapes they had.
Went to the firehouse, they had dalmatian shaped cookies.
The list goes on and on....
The kids were NOT allowed to eat the candy while in school (as per policy), but they are eating it when they get home, KWIM.
The thing that REALLY got me was we had a little one who was allergic (anaphactic too) to peanuts and milk so while all this food was coming in, she had to sit and watch everyone eat it and had nothing.
So happy that experience is done and over with, my heart broke for her.

Food does not need to be used as a teaching tool, especially when you have someone who could have a fatal reaction to it.