Being in the same grade or class; people who are in the same grade. Often used in phrases like '同級の～' or in contrast with '同級生'.
彼は同級の友人と一緒にプロジェクトを担当している。
He is in charge of the project together with a friend from the same grade.
同級の生徒たちは文化祭の準備で忙しかった。
The students in the same grade were busy preparing for the school festival.
高校時代の同級とは今でも連絡を取っている。
I still keep in touch with my classmates from high school.