The Question and Answer section for The Catcher in the Rye is a great resource to ask questions, find answers, and discuss the novel.
Holden leaves schools because he's asked to leave. He has no interest in school, and unless he's interested in the subject matter, he simply won't put in any effort. He isn't kicked out, as much as he flunks out.
According to Stradlater, the most important thing about a composition is that it be descriptive.
Holden is finally told to leave school because of his bad marks. He visits Mr. Spencer, because he liked him and Mr. Spencer had sent him a note requesting a meeting. Holden is, however, repulsed by the meeting. Mr. Spencer shows up in his robe...