So I called the school. First, I have to say I love his school, I love the staff and the teachers. This is not a negative post about any of it, I was just trying to get to the bottom of this mystery. I called with confusion, politely asking if anyone knew where the sticker came from.
There is nothing a kid loves more at Halloween than getting a pillowcase so heavy and full of candy that they have to drag it home, and at the same time, there's nothing a parent loves more than having some sort of healthy benefit from that huge sugar stash. This idea is a win-win.