He works as a junior official.
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.
Compound of 下 (lower) and 級 (grade, rank). The kanji combination directly conveys the meaning of a lower tier in a system.