noun
high-school baseball player
Refers specifically to a student who plays baseball for a high-school team, often with a connotation of dedication and the spirit of high-school baseball in Japan.
彼は甲子園を目指す高校球児だ。
He is a high-school baseball player aiming for Koshien.
球児 is a broader term for a young baseball player, not limited to high school. 高校球児 explicitly specifies the high-school context.
Compound of 高校 (high school) and 球児 (young baseball player).