A teacher assigned it as class reading in sixth grade. It was about a boy who somehow got teleported to another planet full of animals and creatures similar to earth, but just a little different. He finds a tribe, (and a girl he likes?) and helps solve their problems, then one day, finds a can or bottle of Coca Cola buried, realizing he’s been in the future the whole time, and not on another planet. I don’t remember if he makes it back to his time or not.
Lost Classics of Teen Lit: Name That Book! 