You could use lackadaisical in the sense of not really paying attention or caring:
From Oxford English Dictionary (OED):
full of vapid feeling or sentiment; affectedly languishing. Said of persons, their behaviour, manners, and utterances.
The students who study hard usually get better grades, but in contrast the lackadaisical students usually get poor grades.
Or you could try laggard in the sense of lagging behind from OED:
laggard, adj. and n.
Lagging, hanging back, loitering, slow. Chiefly of living things, their actions, and attributes.
One who lags behind; a lingerer, loiterer.
The students who study hard usually get better grades, but in contrast the laggards usually get poor grades.