He is a school Prefect, he has the power to stop their cruel games, but all he does is hide his face behind a textbook and wear masks of pretense while the Slytherin endures his fate.
Just when Snape thought his job was done after delivering the Potion, a snowstorm traps him indoors. Very slight Remus/Severus.
Lupin's reading material, as far as Snape is concerned, is pure rubbish.