The Juniors and Seniors of a high school they are attending. In other words, eleventh and twelfth graders.
Often they are feared the non-upperclassmen, usually freshmen.
Eleventh Grader: YES! We're eleventh graders! That makes us upperclassmen!!!
Ninth Grader: Uh-ohh.... I'm afraid of getting in the way of an upperclassmen....