Muppet High was a planned television series that was never produced. The series was mentioned as having been in the works at The Jim Henson Company in 1990, in the months before Jim Henson's death.
Muppet High was to depict the Muppets as 1950s high school students. A toy company, Presents, produced a line of merchandise in 1990 and 1991 showcasing the Muppets in their 1950s costumes -- a set of PVC figures, two Muppet High dolls, and a set of Muppet High mugs.
Based on the toys, it seems that Kermit would have been a motorcycle-driving teenager with a leather jacket, Miss Piggy would wear a poodle skirt, Fozzie Bear would be a soda jerk, Gonzo would be a bookish nerd, Rowlf would be a varsity football player, and Animal would play the guitar.