When Billy is in kindergarten, watch when the girl tells him he has a mishaped head. When she tells him this she is standing behind him, but when the camera changes, she is magically teleported to the front.
You see, humans have an amazing ability to turn around. Ever figure that when the camera changed, he turned around? Or is Billy Madison too stupid to turn around?
Just for the record, the girl was standing in front of him when she said he has a mis-shaped head, and then it cuts to Billy and then it shows the girl behind Billy, and then she walks in front of him :)
When the girl tells Billy he has a misshapen head she is on his right side, and right after she says that you see her step over in front of Billy and sit down on his left side.