The term is generally used in a fishing vernacular, as the target group usually travel in packs, or schools - like fish, and tend to still be going to school.
Not to be confused with jailbait, which is the correct term for an undersize hardbelly.
Jack: Nah, I'm too old for that shit.
Mick: I snared myself a real nice hardbelly. Had to throw the rest back because they were undersize.
Jack: Next time you get invited to a 21st, YOU RING ME.