I realize this may not occur to anyone as a shoe in for such a course, but I took a class in my sophomore year of college in which we covered Max Brook's World War Z. Almost every other text used in the class was met with mixed enthusiasm (we covered Dune, Neuromancer, Caves of Steel, Electric Sheep, Starship Troopers, etc.) but everyone seemed to love Brooks' work and discussion went fantastically. Any student vaguely familiar with Bush-era political controversy will gain a huge appreciation of how effectively satire can be incorporated in works of science fiction. And everyone loves zombies right now, so it's win-win.
Where Le Guin is concerned... If you dare to subject high school kids to The Left Hand of Darkness, good luck reviving them afterward. I know little about Earthsea, but from what I've heard secondhand, that may be a more viable option for your purposes. If including a female author is what you're looking to do, then go for Mary Shelley, the woman who invented the science fiction novel.
Someone has probably already said it, but show people how wonderful the mind of Tolkien was by giving them The Hobbit, not the trilogy. The Hobbit is the book that made me love to read. As far as I'm concerned, it offers much more memorable people and places in a much tidier package than the drawn-out, song/poem-laden trilogy. One advantage to using LotR, however, would be if you were looking to get into the function of allegory.
For short stories, a nice place to start might be Neil Gaiman's collection Fragile Things.
Dune is awfully hard not to recommend. One of my favorite novels, it wasn't until I read it with others that I started to notice uncanny resemblances to certain modern-day desert conflicts.
And if you get a chance, be sure to fuck their minds up with some Phillip K. Dick and make them laugh with the first installment of Hitchhiker's Guide.