How was Mark a slacker?

Mr. Maxwell doesn't get on at all well with Mark. He sees him as just another one of those slacker rich kids that he cordially loathes. Mark appears disconnected from his immediate learning environment at Hardy Elementary; it just seems like he's marking time until his parents send him to an elite prep school.
Mr. Maxwell's prejudices appear to be confirmed by Mark's bad attitude at school. He makes no effort in class, doesn't try to make any friends, and generally comes across as arrogant on account of his already having attended much better schools. Mark's already earned a place at the best boarding school in the country for sixth grade, and as he'll only be at Hardy Elementary for four months, he figures there's no point in making much of an effort. So his whole demeanor is that of an entitled slacker—a lazy, privileged rich kid who thinks he's much too good for this crummy little school.