After dinner all the New York freshman and most of the older students immediately vanish, inconveniently for Larisa because she really wants to find the kid who apparently had such a nice time with Vanya in the library during work period. He was telling her about it and actually SMILED. 

She makes sure that she's properly handed off Vanya-watching duty to Killian, and then joins up with Justin, who has some errands to do anyway, and checks the library and all the other standard haunts for any New York freshman who might know the girl and her current whereabouts. (Unfortunately she doesn't have a name, Vanya forgot it.)