The 8th Graders are teenagers who attend the eighth grade, they include Scott Tenorman and his friends.
Scott's friends were only seen twice in the episode "Scott Tenorman Must Die". They were seen reading magazines/books with him, and then they helped Scott make the chili with pubes for Cartman. They did not appear again after that episode, though Scott did. They may possibly have appeared in "The Return of the Fellowship of the Ring to the Two Towers".