I teach high school English, and I use this in my 11th grade American lit class. There are so many topics that can be discussed and analyzed, and the characterization is rich and in depth.
It has many positive messages as the characters find ways to gain strength in the face of adversity. It also gives us a glimpse of what psychiatry was like at that time, so provides some historical perspective. I think it's a great example of the finest American lit has to offer, my students always love it, and although there is some risqué content, it's no worse than many classics that are standard fodder for high school English (think of Native Son, read in many high school classes, which contains a graphic description of two men masturbating in a movie theater, as well as a graphic description of Bigger murdering his girlfriend by smashing her head with a brick until it felt like "wet cotton"). I only wish I could follow the book by showing the film, unfortunately the language in the film is even worse than in the book, and I'm not comfortable showing it to 17 year old kids.