When Holden decides to leave Pencey early and go to New York, he stops on his way out of the dormitory building
. . . and then I yelled at the top of my goddamn voice, "Good night, ya morons!"
That is his final goodbye to Pencey Prep. The term "moron" is one Holden frequently uses. He calls Stradlater a moron repeatedly, and later he calls Maurice the bellhop a moron. Although Holden thinks of himself as "dumb" because he can't manage to do his homework assignments and has now gotten kicked out of three prestigious private schools, he is actually exceptionally intelligent, as shown by the fact that he is a whiz at English compositions and has written an entire book full of acute insights and observations.
At least part of Holden's problem is that he is too intelligent. His mind is so active that he finds it impossible to focus on school assignments. Many people seem dumb to him because he has a much higher I.Q., although he doesn't know it.
In some of his short stories, notably in "Franny" and "Zooey," J. D. Salinger wrote about characters with genius I.Q.s and the difficulties their intelligence caused them in human relations. Salinger himself must have had an extremely high I.Q., like his alter ego Seymour Glass. Salinger became a virtual hermit for the last years of his life and gave up publishing, although he may not have given up writing.
In Tne Catcher in the Rye, Holden seems to be looking for people with whom he can communicate and constantly being disappointed. Holden is an ugly duckling. It is interesting that he has no trouble communicating with his little sister Phoebe, but she pretty obviously has a genius I.Q. herself.