Other mistake: The little girl elf pours Santa a mug of hot chocolate, but if you watch closely there is nothing in the mug when he drinks it.
Other mistake: Charlie spray-paints graffiti on the wall of the gymnasium to protest Principal Newman's refusal to decorate the school for Christmas. We see "Newman" in large letters, an equal sign and a decorated Christmas tree with a big red circle and slash across it. The letters have sharp, smooth edges and the tree and ornaments are highly detailed. It is obvious the graffiti was painted with brushes, not spray paint.
Answer: He was getting back at his principal because she hates Christmas.