A student attending junior high school / middle school (typically around ages 12–15).
私の弟は中学生です。
My younger brother is a junior high school student.
中学生になると部活動が本格化することが多い。
Club activities often become more serious when students enter junior high school.
中学生向けの参考書を買いに本屋へ行った。
I went to the bookstore to buy reference books for junior high students.