same school year; same grade
Refers to being in the same year/grade at school. Often used in contexts like 同学年の友達 (friend in the same grade) or 同学年で一番 (best in the grade).
He and I were in the same school year, and we often studied together.
Among the students in the same grade, she is the tallest.
Compound of 同 (same) + 学年 (school year/grade).