Synecdoche is a figure of speech which expresses either more or less than it literally denotes. By it we give to an object a name which literally expresses something more or something less than we intend.
An example of this figure of speech appears on page 4 of the book when Holden says that Pencey is full of crooks because he was robbed. He suggests that the school's student body are all crooks, when in fact there is only one crook, the person who stole his camel hair coat right out of his room.
"Pencey was full of crooks. Quite a few guys came from these wealthy families, but it was full of crooks anyway. The more expensive a school is, the more crooks it has, I'm not kidding." (Salinger)
Holden speaks of the whole school as being full of crooks, when he means that he knows of one crook, the person who took his coat. He exaggerates the issue by suggesting that he is surrounded by crooks to make the event about his coat seem very important, and to denigrate Pencey since he being kicked out he has to find fault with the school that no longer wants him as a student.