I've been trying to remember the name of a story I read in elementary school. I forget the exact plot but a group of kids somehow end up on an alien spaceship. They eventually end up on an alien planet where the help the natives. After some adventures, they get back to earth where they discover they have been replaced with aliens who look like them.
The story was divided into 3 parts and spread across 3 different anthology books. I think the point was to read one section in each grade for 3 grades.