upper grades of elementary school
Typically refers to 5th and 6th grades, sometimes including 4th grade. Used in school settings to distinguish from lower (低学年) and middle (中学年) grades.
Upper-grade students participate in committee activities.
この本は高学年向けです。
This book is aimed at upper elementary grades.
Once you reach 5th grade, you become an upper-grader.
This book is written for upper-grade elementary students.