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third year of junior high school; third-year junior high school student
彼は中三の受験生だ。
He is a third-year junior high school student preparing for entrance exams.
中三になると部活と勉強の両立が難しくなる。
When you reach the third year of junior high, balancing club activities and studying gets harder.
中三のクラスで合唱コンクールの練習が始まった。
Practice for the choral contest has started in the third-year junior high class.