Campus slang from Hong Kong/Taiwan that spread on the mainland in the 2000s. Female counterpart: 班花 ('class flower'). Higher levels: 校草 (school heartthrob), 系草 (faculty heartthrob).
memory
班 (class) + 草 (grass) - but here 草 stands as a 'plant' metaphor for a guy. Pair: 草 (boy) vs 花 (girl).