Intro
Do we really need to tell you what Harry Potter is about? Marxists would love the central message: the world is bigger than school! Get out there and wave those wands!
When Harry, Ron, and Hermione go into battle against Voldemort and all his little hooded Death-Eaters, we have a major case of the class struggle. It's the evil exploiters against the pimply meritocrats (they're poor, but they're talented).
The outcome is pretty much what the Iron Laws of History say it's gonna be.
Quote
"Silence," said Voldemort. […] "Not content with corrupting and polluting the minds of Wizarding children, last week Professor Burbage wrote an impassioned defense of Mudbloods in the Daily Prophet. Wizards, she says, must accept these thieves of their knowledge and magic. The dwindling of the purebloods is, says Professor Burbage, a most desirable circumstance. . . . She would have us all mate with Muggles […] or, no doubt, werewolves […]."
Analysis
The evil dark wizard Voldemort is basically a big ol' jerk. This is magical classism: aristocrats (the purebloods) don't want to mix with the common folk (the Mudbloods). According to Voldemort, a talented Mudblood with magical powers must have stolen his abilities from somewhere, because only certain families are born talented.
Voldemort is clearly no democrat.
A Marxist would say that the Harry Potter novels express the values of bourgeois society. Aristocrats, from the bourgeois point of view, are evil and exclusionary. Harry and his little chums are good bourgeois kids—they triumph through individual talent and cooperation. So far, so realist.