He works as a junior official.
下級生は上級生を敬うべきだ。
Lower-grade students should respect upper-grade students.
上級 means 'upper grade' or 'advanced level', the direct opposite of 下級 which means 'lower grade' or 'junior'.
低級 usually implies poor quality or vulgarity, whereas 下級 neutrally describes a lower rank or grade in a hierarchy.
student in a lower grade; underclassman
lower court
subordinate post; lower-ranking position
inferior good
low-class citizen; second-class citizen
lower court
Compound of 下 (lower) and 級 (grade, rank). The kanji combination directly conveys the meaning of a lower tier in a system.