Although Holden Caulfield is only sixteen years old, he has been expelled from three prep schools. It would appear that he has had virtually no high school education. This must be making it increasingly difficult for him to keep up with his schoolwork. By his age he should be entering the 11th grade, but he has missed out on much of the instruction he should have received in the 9th and 10th grades. The only reason he is able to write what is represented in Salinger's novel as his autobiography is that Holden likes to read. He is pretty much self-educated, and he does all right in English classes because he can not only write competent English but he is obviously very intelligent and not inhibited about expressing himself. His English teachers appreciate him, but the other teachers, such as Mr. Spencer, consider him a problem student. Holden is only self-educated to some degree in the kind of literature he likes to read. He is probably quite weak in such subjects as mathematics, science, and history.
Once Holden gets on that "slippery slope" by failing to absorb the subject matter for one year, probably due to trauma and mental health issues, he is forced to play "catch-up" for the rest of his academic career. The instruction and the books tend to seem more and more opaque, and also more and more esoteric and inconsequential. School becomes a rather strange and mysterious place.
Holden Caulfield is a lonely outsider. He has been to three prep schools already, and when he goes on to a fourth school he is bound to be a lonely outsider there.