J.V. Jocks are a clique at North Shore High School. They are defined as such by Janis Ian in a map of the North Shore Cafeteria intended for Cady Heron's use. When Janis gives a verbal description of the map, the film does not show this clique at that time.
In high school, J.V. sports teams are like "second" or "reserve" athletic teams, which are designed for students that aren't athletic enough for the Varsity Teams. This means that the people in the clique are probably "lesser jocks"