upper grades of primary school (sixth, fifth, and sometimes fourth grades)
高学年が1年生を助ける。
Upper-grade students help first-graders.
高学年になると読書が難しくなる。
When students reach the upper grades, reading becomes more challenging.
高学年の児童は、外遊びより宿題に時間を費やす。
Children in the upper grades spend more time on homework than on outdoor play.