When aurors from the Ministry of Magic show up at Hogwarts to try to blackmail Dumbledore into fighting Grindelwald for them, they interrupt him teaching a Defence Against the Dark Arts class. He happens to be going toe-to-toe with a young, dark-haired student who isn't afraid to mouth off to the aurors when they try to haul his "favourite" teacher away.
McGonagall proceeds to call after the boy, saying something like "come along now, McLaggen." As in Cormac McLaggen? We assume so! As you'll recall, Cormac is a member of the Slug Club and also plays for Gryffindor's quidditch team (which is how he gets the opportunity to thwack Harry over the head with a beater's bat). Despite the fact he's clearly a tool, his family is also "big in the Ministry," so he's used to getting a pass. Now we can see where he gets his cocky attitude from!