My Teacher is an Alien — Bruce Coville

SIXTH GRADE IS JUST ... OUT OF THIS WORLD
Susan Simmons can tell that her new substitute teacher is really weird. But she doesn't know how weird until she catches him peeling off his face -- and she realizes that "Mr. Smith" is really an alien!

At first no one will believe her -- except Peter Thompson, the class brain. When Peter and Susan discover Mr. Smith's horrible plans for their classmates, they know they have to act fast. Only they can get rid of their extraterrestrial visitor -- and save the rest of the sixth grade class from a fate worse than math tests!