Upperclassman

Someone who is older than you, or higher than you. Translated it means senpai in japanese.

"He greatly respects Mamoru, who is an upperclassman at his school."

upperclassman

Any student in a high school other than a freshman. These students are usually pretty normal, but for some of them, the evolution from frosh to non-frosh makes them into total jackasses. These jackass-upperclassmen should be avoided by freshmen at all costs.

Upperclassman: Hey, you're a freshman, right?
Freshman: Uh ... yeah?
Upperclassman: Go clean the floor with your tongue.