When Billy is in first grade they are making snowmen out of cotton, if you look Billy's is pretty sloppy. then when he is flirting with one of his classmates mom, Mrs. Lippy comes out with his snowman, improved. This snowman is neat.
Ok Blah - realize this is a movie. No, Mrs. Lippy or any other kids made him a new one. Why would they do that unless they made it obvious in the movie. Mrs. Lippy says that he forgot his stuff, she didn't add on "Oh, and we made you a new snowman because you're was sloppy."
You guys are right, this could very well be a slip-up, but I highly doubt that they would miss something that obvious. You have to remember this is a comedy. They may have done that on purpose.