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.
As a senior student at the school, he often looks after the younger students.
When you become a senior, you get busy with career planning.
上級生 means 'upperclassman' in general, while 最上級生 specifically refers to the highest grade level.
最上級 means 'highest grade' or 'superlative' and can be used in various contexts, not limited to students.
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.