“Slumming” is an old expression that doesn’t have any racial content. There have been slums of all descriptions, occupied by all sorts of people. It refers, usually ironically, to a person spending time with those in a lower social position. in this case, he might ask himself, “What the hell is this girl who is in the honors/AP crowd doing slumming it in this class? She doesn’t belong here.” Similarly, a kid in a third-string basketball team might ask, if he showed up to play, “What is he doing, slumming it here with us?”
No intention on my part to be offensive.