How is Mr. Spencer significant?

In the opening chapters of The Catcher in the Rye, Holden Caulfield describes the school and several of the students, but the real author, J. D. Salinger, must have felt it would seem incomplete if his narrator did not give some description of the teachers as well. This probably explains why Holden goes to say goodbye to Mr. Spencer. The elderly teacher actually is, in one respect, a representative of the entire faculty at Pencey. Mr. Spencer is speaking for all of Holden's teachers when he explains in an apologetic manner why he had to give the boy an "F" as his final grade. The other teachers probably felt the same way as Mr. Spencer. They could not help feeling that they might have done something—and should have done something—to "motivate" this bright and talented but totally indifferent young student. They know it will look bad for Holden when he goes home to face his parents, who were paying a lot of money to send their son to one of the most prestigious prep schools in the country, one that supposedly has been "molding young boys into splendid, clear-thinking young men" since 1888 and runs magazine ads showing one of these splendid, clear-thinking young men leaping over a fence on horseback. It looks bad for Holden, but it also looks bad for the teachers, the headmaster, and the school. The parents might ask, "Why didn't you mold Holden into a splendid, clear-thinking young man instead of sending him home with a report card full of F's?"
Mr. Spencer is not reprimanding Holden for his misbehavior so much as he is apologizing and trying to justify his own perceived failure. Holden is kind of a super-dropout. He has flunked out of two prestigious prep schools before, and now he is flunking out of the third. Three times is a charm. He probably will not ever be going back to another prep school. The teachers could not have motivated this problem student, but nevertheless they feel somewhat embarrassed because they sense that some of the blame will attach to them. That is why Mr. Spencer, speaking in a sense for all the teachers, asks:
"Do you blame me for flunking you, boy?"
It seems ironic that Holden ends up consoling his old teacher:
Well, you could see he really felt pretty lousy about flunking me. So I shot the bull for a while. I told him I was a real moron, and all that stuff. I told him how I would've done exactly the same thing if I'd been in his place, and how most people don't appreciate how tough it is being a teacher. That kind of stuff. The old bull.
Holden and Mr. Spencer seem to change places, which is underscored by Holden saying, "I would've done exactly the same thing if I'd been in his place."