noun
senior student; student in the highest grade
Refers to a student in the final year or highest grade level at a school, such as a third-year junior high student or a senior in high school.
彼は学校の最上級生として、下級生の面倒をよく見ている。
As a senior student at the school, he often looks after the younger students.
最上級生になると、進路のことで忙しくなる。
When you become a senior, you get busy with career planning.
Compound of 最上 (highest) + 級生 (grade student). The exact historical derivation is uncertain, but the word follows a regular pattern of combining grade-related terms.