Undergraduate students who are juniors or seniors (usually third and fourth year, or more). You may sometimes hear it applied to sophomores as well, or any students not in their first year.

What grades are considered upperclassmen?

The definition of an upperclassman is a junior or senior in high school or college. A senior in high school who is about to graduate is an example of an upperclassman. A student in the junior or senior class of a high school or college. (US) A junior or senior student in a school or college.

What does it mean to be an upperclassmen?

: a member of the junior or senior class in a school or college.