The "rejects" are a group of neglected kids that are usually found in the back of the cafeteria of the Skool. In the episode "Bestest Friend", Zim attempts to befriend some of his classmates in the hopes that it will make him appear more human.
The "rejects", who apparently have no friends, are Keef, Matthew P, Mathers lll, Gretchen, Melvin, and Dirge. These were the people that Zim tried to befriend. Keef won Zim's makeshift contest and became Zim's "bestest friend," although the friendship lasts for only half of the episode.
Despite sitting together at the same lunch table, the "rejects" were rarely seen together in the other episodes. It can be implied that none of them are actually friends, but rather they merely sit with each other.
One of the rejects, Dirge, clamed to be born with webbed fish toes, which made Zim change his mind about befreinding him. Later in the episode, after Keef claimed that the squirrel was Zim, he bends down to untie his shoes, claiming it was "really neat". This made one of the other rejects, Matthew P. Mathers, run away screaming.