Well, to be taken seriously, you'd need to ignore the fact that it's a high school lit class. It's tough to say what to choose, since a good lit class should have a 'theme' other than two broad genres. I'd focus on novels / stories that are plot-driven and had more to offer - things that could provide insight into literary trends / techniques.
Orwell is taught anyway in mainstream lit (at least when I was in high school) so I'd pass on him.
I'd second Vance, especially The Eyes of the Overworld / Cugel the Clever. It's a great modern picaresque.
Vonnegut certainly. Hard to say which I'd choose. Probably stay predictable and go with Slaughterhouse Five. He employs some interesting narrative tricks.
It's too bad the Gormenghast novels are prohibitive in length - they're some of the finest fantasy written and avoid the typical genre cliches.